Electrode structure for solid polymer fuel cell, its production method, and solid polymer fuel cell

ABSTRACT

The present invention provides an electrode structure having a pair of electrode catalyst layers and a polymer electrolyte membrane held between both the electrode catalyst layers. The polymer electrolyte membrane contains 5% or more by weight of the water coordinated to protons of sulfonic acid groups. The polymer electrolyte membrane comprises a fluorine-containing ion conducting polymer. The ratio of the fluorine content in the above polymer electrolyte membrane to the fluorine content in the above electrode catalyst layer is within the range of from 0.2 to 2.0. The polymer electrolyte membrane is a sulfonate of a copolymer general formulas (1) and (2). The electrode catalyst layer contains platinum within the range of from 0.01 to 0.8 mg/cm 2 , and the average particle diameter of a carbon particle as a carrier is within the range of from 10 to 100 nm. The polymer electrolyte membrane is produced by forming a membrane from the above sulfonate solution and drying the obtained membrane. The membrane contains from 3 to 15% by weight of a solvent after drying. The electrode structure is produced by applying an electric current of 0.1 A/cm 2  or higher for 5 hours or more in a humidified environment at a relative humidity of 60% or more. The electrode structure constitutes a fuel cell, which generates power when oxidizing gas is supplied to the one side of the electrode structure and reducing gas to the other side.

TECHNICAL FIELD

[0001] The present invention relates to an electrode structure used fora polymer electrolyte fuel cell, a method for producing the same, and apolymer electrolyte fuel cell, which uses the electrode structure.

BACKGROUND ART

[0002] The petroleum source is beginning to dry up, and at the sametime, environmental problems such as global warming due to theconsumption of fossil fuel have increasingly become serious. Thus, afuel cell receives attention as a clean power source for electric motorsthat is not accompanied with the generation of carbon dioxide. The abovefuel cell has been widely developed, and some fuel cells have becomecommercially practical. When the above fuel cell is mounted in vehiclesand the like, a polymer electrolyte fuel cell comprising a polymerelectrolyte membrane is preferably used because it easily provides ahigh voltage and a large electric current.

[0003] As an electrode structure used for the above polymer electrolytefuel cell, there has been known an electrode structure, which comprisesa pair of electrode catalyst layers comprising a catalyst such asplatinum supported by a catalyst carrier such as carbon black that isformed by integrating by an ion conducting polymer binder, a polymerelectrolyte membrane capable of conducting ions sandwiched between theelectrode catalyst layers, and a backing layer laminated on each of theelectrode catalyst layers. When a separator acting also as a gas passageis further laminated on each of the electrode catalyst layers, the aboveelectrode structure constitutes a polymer electrolyte fuel cell.

[0004] In the above polymer electrolyte fuel cell, one electrodecatalyst layer is defined as a fuel electrode, and the other electrodecatalyst layer is defined as an oxygen electrode. Now, reducing gas suchas hydrogen or methanol is introduced into the fuel electrode throughthe above backing layer, whereas oxidizing gas such as air or oxygen isintroduced into the oxygen electrode through the above backing layer. Bythis action, on the above fuel electrode side, protons are generatedfrom the above reducing gas by the action of a catalyst contained in theabove electrode catalyst layer. Then, the protons transfer to theelectrode catalyst layer on the above oxygen electrode side through theabove polymer electrolyte membrane. Thereafter, the protons are reactedwith the above oxidizing gas introduced into the oxygen electrode by theaction of the above catalyst contained in the electrode catalyst layeron the above oxygen electrode side, so as to generate water. Thus, theabove fuel electrode is connected to the above oxygen electrode throughusing a conductor, so as to obtain electric current.

[0005] Previously, in the above electrode structures, aperfluoroalkylene sulfonic acid polymer (e.g., Nafion (trade name) fromDuPont) has been widely used for the above polymer electrolyte membrane.The perfluoroalkylene sulfonic acid polymer is sulfonated, andaccordingly it has an excellent proton conductivity. Moreover, thecompound also has a chemical resistance as a fluorocarbon resin.

[0006] However, the compound is inconvenient in that it is extremelyexpensive.

DISCLOSURE OF THE INVENTION

[0007] It is an object of the present invention to solve such aninconvenience and to provide an inexpensive electrode structure for apolymer electrolyte fuel cell having an excellent power generationefficiency.

[0008] Moreover, it is another object of the present invention toprovide a method for producing the above electrode structure for apolymer electrolyte fuel cell.

[0009] Furthermore, it is another object of the present invention toprovide a polymer electrolyte fuel cell in which the above electrodestructure is used.

[0010] To achieve the above objects, the electrode structure for apolymer electrolyte fuel cell (hereinafter abbreviated as an electrodestructure at times) of the present invention comprises a pair ofelectrode catalyst layers and a polymer electrolyte membrane sandwichedbetween both the electrode catalyst layers, characterized in that theabove polymer electrolyte membrane is a sulfonate of a hydrocarbon-basedpolymer comprising a main chain, in which two or more benzene rings arebound to one another, directly or through the medium of a divalentorganic group.

[0011] Examples of the above hydrocarbon-based polymer may includecompounds such as polyether ether ketone or polybenzimidazole, andrigid-rod polyphenylene disclosed in U.S. Pat. No. 5,403,675. Thesulfonate of the rigid-rod polyphenylene disclosed in the abovedescription comprises, as a main ingredient, a polymer obtained bypolymerizing an aromatic compound having a phenylene chain.

[0012] Moreover, examples of the above hydrocarbon-based polymer mayalso include a copolymer consisting of a first repeating unitrepresented by the following general formula (1) and a second repeatingunit represented by the following general formula (2):

[0013] wherein A represents an electron attracting group, B representsan electron releasing group group, n is an integer of 0 or 1, and abenzene ring includes a derivative thereof, and

[0014] wherein A represents an electron attracting group, B representsan electron releasing group group, Y represents —C(CF₃)₂— or —SO₂—, anda benzene ring includes a derivative thereof.

[0015] It should be noted that the term “electron attracting group” isused in the present description to mean a divalent group such as —CO—,—CONH—, —(CF₂)p- (wherein p is an integer of 1 to 10), —C(CF₃)₂—, —COO—,—SO— or —SO₂—, in which the Hammett substituent constant is 0.06 orgreater in the meta position of a phenyl group and it is 0.01 or greaterin the para position thereof. It should be also noted that the term“electron releasing group group” is used herein to mean a divalent groupsuch as —O—, —S—, —CH═CH—, or —C≡C—.

[0016] The above hydrocarbon-based polymer contains no, or a reducedamount of fluorine in a molecular structure thereof. Accordingly, whenthe sulfonate of the above hydrocarbon-based polymer is used as amaterial for the above polymer electrolyte membrane, an inexpensiveelectrode structure having an excellent power generation efficiency canbe obtained.

[0017] When an electrode structure comprising a polymer electrolytemembrane composed of the above hydrocarbon-based polymer is activated ata lower temperature of 0° C. or lower, if water generated in the area ofthe above oxygen electrode and water contained in the above polymerelectrolyte membrane freeze, a sufficient ion conductivity may not beobtained at times.

[0018] Thus, in the first aspect, the electrode structure of the presentinvention is characterized in that the above polymer electrolytemembrane contains 5% or more by weight of the coordinated water of aproton of a sulfonic acid group based on the total weight of the polymerelectrolyte membrane.

[0019] It is known that a high polymer comprising the above sulfonicacid group contains coordinated water in the proton of the sulfonic acidgroup. The coordinated water does not freeze even below the freezingpoint. Thus, since the electrode structure for a polymer electrolytefuel cell of the present invention contains at least 5% or more byweight of the coordinated water based on the total weight of the abovepolymer electrolyte membrane, even when it is activated at a lowtemperature of 0° C. or lower, it keeps water necessary for ionconduction, so that it can obtain an excellent ion conductivity.

[0020] Rigid-rod polyphenylene disclosed in U.S. Pat. No. 5,403,675 isexcellent in ion conductivity and creep resistance in a high temperatureenvironment, but it is insufficient in oxidation stability.

[0021] Thus, in the second aspect, the electrode structure of thepresent invention is characterized in that the above polymer electrolytemembrane comprises an ion conducting polymer containing fluorine in amolecular structure thereof, and in that the ratio (Y/X) of the contentof fluorine in the above polymer electrolyte membrane (Y) to the contentof fluorine in the above electrode catalyst layer (X) is within therange between 0.2 and 2.0.

[0022] The term “the content of fluorine in the above electrode catalystlayer (X)” is used herein to mean the weight ratio of fluorine containedin the molecular structure of the above ion conducting polymer binderbased on the total weight of the above electrode catalyst layer. On theother hand, the term “the content of fluorine in the above polymerelectrolyte membrane (Y)” is used herein to mean the weight ratio offluorine contained in the molecular structure of the above ionconducting polymer based on the total weight of an ion conductingpolymer constituting the above polymer electrolyte membrane.

[0023] The above Y/X is within the range between 0.2 and 2.0 in theelectrode structure in the above second aspect, so that the electrodestructure can obtain a good oxidation stability, as well as a good creepresistance when a fuel cell comprising the electrode structure isactivated at a high temperature. If the above Y/X is less than 0.2, theelectrode structure cannot obtain a sufficient oxidation stability. Ifthe Y/X exceeds 2.0, the electrode structure cannot obtain a sufficientcreep resistance when a fuel cell comprising the electrode structure isactivated at a high temperature.

[0024] The electrode structure in the above second aspect ischaracterized in that the above polymer electrolyte membrane is asulfonate of a copolymer of the first repeating unit represented bygeneral formula (1) and the second repeating unit represented by generalformula (2), and in that the first repeating unit or the secondrepeating unit contains fluorine.

[0025] Herein, sulfonation takes place only on a benzene ring to whichno electron attracting group binds. Accordingly, when a copolymer of thefirst repeating unit represented by general formula (1) with the secondrepeating unit represented by general formula (2) is sulfonated, nosulfonic acid group is introduced either into any benzene ring withinthe first repeating unit, which belongs to the main chain of thecopolymer, or any benzene ring within the second repeating unit, butbenzene rings within the side chain of the first repeating unit may besulfonated. Thus, in the above copolymer, the molar ratio between thefirst repeating unit and the second repeating unit is adjusted tocontrol the amount of the introduced sulfonic acid group, so as toobtain a polymer electrolyte membrane having an excellent ionconductivity.

[0026] In the above second aspect, in order to obtain a copolymerconstituting the above polymer electrolyte membrane, either one of, orboth of the first repeating unit and the second repeating unit shouldcontain fluorine in their molecular structure. The combination of2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone represented by thefollowing formula (3) and2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropanerepresented by the following formula (4) can be an example of thecombination of the first repeating unit and the second repeating unit:

[0027] Since rigid-rod polyphenylene disclosed in U.S. Pat. No.5,403,675 has low toughness, when the sulfonate of the rigid-rodpolyphenylene is used for a polymer electrolyte membrane to constitutean electrode structure, the membrane is easily torn off. In addition,the rigid-rod polyphenylene disclosed in the above description cannotobtain a sufficient ion conductivity at times.

[0028] Thus, in the third aspect, the electrode structure of the presentinvention is characterized in that the above polymer electrolytemembrane is a sulfonate of a copolymer of the first repeating unitrepresented by general formula (1) and the second repeating unitrepresented by general formula (2), and in that the above electrodecatalyst layer contains, as a catalyst, platinum within the rangebetween 0.01 and 0.8 mg/cm², and the average diameter of a carbonparticle as a carrier supporting the platinum is within the rangebetween 10 and 100 nm.

[0029] In the above copolymer, as described above, the amount of theintroduced sulfonic acid group can be controlled by adjusting the molarratio between the first repeating unit and the second repeating unit.Hence, by controlling the amount of the introduced sulfonic acid group,a polymer electrolyte membrane having an excellent ion conductivity andtoughness can be obtained.

[0030] In the above third aspect, a specific example of a monomer usedas the first repeating unit may include2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone represented by the aboveformula (3). Moreover, specific examples of a monomer used as the secondrepeating unit may include2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropanerepresented by the above formula (4), and2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]sulfone represented by thefollowing formula (5):

[0031] The sulfonate of the above copolymer is used for the abovepolymer electrolyte membrane in the electrode structure in the abovethird aspect, so that the electrode structure can be easily produced andthe produced electrode structure can obtain an excellent powergeneration efficiency.

[0032] In the electrode structure in the above third aspect, theelectrode catalyst layers sandwiching the above polymer electrolytemembrane contain, as a catalyst, platinum within the range between 0.01and 0.8 mg/cm², and the average diameter of a carbon particle as acarrier supporting the platinum is within the range between 10 and 100nm. By these features, the electrode structure can obtain a furtherexcellent power generation efficiency.

[0033] If the content of the above platinum is less than 0.01 mg/cm², asufficient power generation efficiency might not be obtained, and if itexceeds 0.8 mg/cm², the above platinum acts as a negative catalyst, anddeterioration of the copolymer constituting the above polymerelectrolyte membrane might be promoted.

[0034] If the average diameter of the above carbon particle is less than10 nm, the dispersion of the above platinum is inhibited, and if itexceeds 100 nm, activation overvoltage increases. In both cases, asufficient power generation efficiency might not be obtained.

[0035] In the electrode structure in the above third aspect, in order tocontrol the amount of the introduced sulfonic acid group so as to setion conductivity and toughness in a preferred range, the copolymerconstituting the above polymer electrolyte membrane preferably comprises10 to 80 mol % of the above first repeating unit and 90 to 20 mol % ofthe above second repeating unit. If the copolymer comprises less than 10mol % of the above first repeating unit and more than 90 mol % of theabove second repeating unit, the amount of a sulfonic acid groupintroduced into the copolymer decreases, and so a sufficient ionconductivity might not be obtained. In contrast, if the copolymercomprises more than 80 mol % of the above first repeating unit and lessthan 20 mol % of the above second repeating unit, the amount of asulfonic acid group introduced into the copolymer increases, and so asufficient toughness might not be obtained.

[0036] Moreover, in the electrode structure in the above third aspect,in order to set ion conductivity and toughness in a preferred range, thesulfonate of the copolymer constituting the above polymer electrolytemembrane preferably contains a sulfonic acid group within the rangebetween 0.5 and 3.0 mg equivalent/g. If the amount of a sulfonic acidgroup contained in the above copolymer is less than 0.5 mg equivalent/g,a sufficient ion conductivity might not be obtained. If it exceeds 3.0mg equivalent/g, a sufficient toughness might not be obtained.

[0037] In the fourth aspect, the electrode structure of the presentinvention is characterized in that the above polymer electrolytemembrane is produced by forming a membrane from a solution obtained bydissolving into a solvent a sulfonate of a copolymer of the firstrepeating unit represented by general formula (1) and the secondrepeating unit represented by general formula (2) and drying theobtained membrane, and in that the polymer electrolyte membrane contains3 to 15% by weight of said solvent after drying it.

[0038] The above polymer electrolyte membrane is produced by forming amembrane from a solution obtained by dissolving into a solvent thesulfonate of the above copolymer according to cast method or the like,and drying the obtained membrane. Herein, the above polymer electrolytemembrane contains the above solvent within the range of 3 to 15% byweight after drying, and thereby it can obtain a particularly excellenttoughness.

[0039] When the content of the above solvent is less than 3% by weightafter drying, the above polymer electrolyte membrane cannot obtain asufficient toughness, but when it exceeds 15% by weight, the membranecannot obtain a sufficient power generation efficiency.N-methylpyrrolidone is suitable as a solvent to obtain an electrodestructure having an excellent power generation efficiency.

[0040] In the electrode structure in the above fourth aspect, in orderto control the amount of the introduced sulfonic acid group so as to setion conductivity and toughness in a preferred range, the copolymerconstituting the above polymer electrolyte membrane preferably comprises10 to 80 mol % of the above first repeating unit and 90 to 20 mol % ofthe above second repeating unit. If the copolymer comprises less than 10mol t of the above first repeating unit and more than 90 mol % of theabove second repeating unit, the amount of a sulfonic acid groupintroduced into the copolymer decreases, and so a sufficient ionconductivity might not be obtained. In contrast, if the copolymercomprises more than 80 mol % of the above first repeating unit and lessthan 20 mol % of the above second repeating unit, the amount of asulfonic acid group introduced into the copolymer increases, and so asufficient toughness might not be obtained.

[0041] Moreover, in the electrode structure in the above fourth aspect,in order to set ion conductivity and toughness in a preferred range, thecopolymer constituting the above polymer electrolyte membrane preferablycontains a sulfonic acid group within the range between 0.5 and 3.0 mgequivalent/g. If the amount of a sulfonic acid group contained in theabove copolymer is less than 0.5 mg equivalent/g, a sufficient ionconductivity might not be obtained. If it exceeds 3.0 mg equivalent/g, asufficient toughness might not be obtained.

[0042] Furthermore, when the polymer electrolyte membrane comprising theabove hydrocarbon-based polymer is intended to be integrated with theabove pair of electrode catalyst layers by sandwiching it between thelayers, a sufficient adhesion might not be attained between the abovepolymer electrolyte membrane and the electrode catalyst layers. If theadhesion between the above polymer electrolyte membrane and the aboveelectrode catalyst layers is low, protons are inhibited from passingthrough the interface between the polymer electrolyte membrane and theelectrode catalyst layer in the electrode structure, and thereby a goodpower generation efficiency cannot be obtained.

[0043] Thus, the method for producing an electrode structure of thepresent invention is characterized in that it comprises the steps of:sandwiching a polymer electrolyte membrane by a pair of electrodecatalyst layers to integrate both the electrode catalyst layers and thepolymer electrolyte membrane, so as to form an electrode structure; andapplying an electric current of 0.1 A/cm² or higher to the electrodestructure for 5 hours or more in a humidified environment at a relativehumidity of 60% or more.

[0044] According to the production method of the present invention, apolymer electrolyte membrane is sandwiched between a pair of electrodecatalyst layers for integration, so as to form an electrode structure,and thereafter, an electric current of 0.1 A/cm² or higher is applied tothe electrode structure for 5 hours or more in a humidified environmentat a relative humidity of 60% or more. By this process, the generatedprotons penetrate into the above polymer electrolyte membrane on thefuel electrode side of the above electrode structure. Moreover, by thepenetration of the protons, water transfers from the oxygen electrodeside into the above polymer electrolyte membrane.

[0045] As a result, the electrode structure adopts a structure such thateach electrode catalyst layer penetrates into the polymer electrolytemembrane side at the interface between the catalyst layer and themembrane, thereby improving the adhesion between each electrode catalystlayer and the polymer electrolyte membrane.

[0046] The phenomenon that each of the above electrode catalyst layerspenetrates on the above polymer electrolyte membrane side can beconfirmed by measuring the length of the interface between eachelectrode catalyst layer and the polymer electrolyte membrane, using amap meter and the like. In the electrode structure produced by theprocess according to the present invention, in order to improve theadhesion between each of the above electrode catalyst layers and theabove polymer electrolyte membrane, the actual length of the interfaceis preferably 15% or more longer than the slant distance between anygiven two points on the interface between each electrode catalyst layerand the polymer electrolyte membrane (the actual interface length/theslant distance≧1.15).

[0047] For the measurement of the length of the above interface, it isdesirable to set the slant distance between the above any given twopoints at 10 μm or longer and to make the average of the resultsobtained by measuring any given 7 or more slant distances.

[0048] According to the production method of the present invention, inorder that protons generated in the above fuel electrode easilytransfer, it is necessary to apply an electric current to the aboveelectrode structure in a humidified environment at a relative humidityof 60% or more. In an environment where a relative humidity is less than60%, when an electric current is applied to the above electrodestructure, the phenomenon that the above electrode catalyst layerpenetrates on the polymer electrolyte membrane side hardly occurs.

[0049] In addition, according to the production method of the presentinvention, an electric current of 0.1 A/cm² or higher, preferably of 0.1to 2 A/cm² is applied to the above electrode structure for 5 hours ormore, preferably for 8 hours or more, in the above humidifiedenvironment.

[0050] When an electric current of less than 0.1 A/cm² is applied, theeffect of improving the adhesion between the above electrode catalystlayer and the above polymer electrolyte membrane cannot be obtained. Incontrast, when an electric current of more than 2 A/cm² is applied, thedeterioration of the electrode structure occurs. When an electriccurrent is applied for shorter than 5 hours, the effect of improving theadhesion between the above electrode catalyst layer and the abovepolymer electrolyte membrane cannot be obtained.

[0051] The production method of the present invention can be appliedeven in a case where the above polymer electrolyte membrane is aperfluoroalkylene sulfonic acid polymer, but it can be preferablyapplied in a case where the above polymer electrolyte membrane is asulfonate of a hydrocarbon-based polymer comprising a main chain, inwhich two or more benzene rings are bound to one another, directly orthrough the medium of a divalent organic group.

[0052] Examples of the above hydrocarbon-based polymer may includepolyether ether ketone, polybenzimidazole, and rigid-rod polyphenylenedisclosed in U.S. Pat. No. 5,403,675. In order to obtain a polymerelectrolyte membrane that is excellent in ion conductivity andmechanical strength, however, the hydrocarbon-based polymer ispreferably a copolymer, which comprises a main chain comprising thefirst repeating unit represented by general formula (1) and the secondrepeating unit represented by general formula (2).

[0053] Since the second repeating unit represented by general formula(2) comprises a main chain having a flexible structure, it improves themechanical strength of the above copolymer such as toughness.

[0054] In order to control the amount of the introduced sulfonic acidgroup so as to set the ion conductivity and mechanical strength of thecopolymer in a preferred range, the molar ratio between the firstrepeating unit and the second repeating unit is preferably adjusted inthe range of 10 to 80 mol % of the first repeating unit and 90 to 20 mol% of the second repeating unit. When the copolymer comprises less than10 mol % of the first repeating unit and more than 90 mol % of thesecond repeating unit, the amount of a sulfonic acid group introducedinto the copolymer is insufficient, and so the above polymer electrolytemembrane has a low ion conductivity. In contrast, when the copolymercomprises more than 80 mol % of the first repeating unit and less than20 mol % of the second repeating unit, the above polymer electrolytemembrane cannot have a sufficient mechanical strength.

[0055] The above copolymer preferably has a polymer molecular weight of10,000 to 1,000,000 at a weight-average molecular weight shown usingpolystyrene conversion. If the above polymer molecular weight is lessthan 10,000, a mechanical strength that is preferable as a polymerelectrolyte membrane might not be obtained. If it exceeds 1,000,000,when the polymer is dissolved in a solvent to form a membrane, thedissolubility decreases or the viscosity of the solution increases, andthereby it becomes difficult to treat the polymer.

[0056] Moreover, the above copolymer is sulfonated preferably such thatit contains a sulfonic acid group within the range between 0.5 and 3.0mg equivalent/g. If the obtained sulfonate contains less than 0.5 mgequivalent/g of sulfonic acid group, it cannot obtain a sufficient ionconductivity. If the content of a sulfonic acid group exceeds 3.0 mgequivalent/g, a sufficient toughness cannot be obtained, and it becomesdifficult to treat the sulfonate during the production of an electrodestructure.

[0057] Both the electrode structure in each aspect of the presentinvention and the electrode structure obtained by the production methodof the present invention constitute a polymer electrolyte fuel cell,which generates electric power, when oxidizing gas is supplied to theone side of the above electrode structure and reducing gas to the otherside.

BRIEF DESCRIPTION OF THE DRAWINGS

[0058]FIG. 1 is an illustrative sectional view of the electrodestructure for a polymer electrolyte fuel cell of the present invention;

[0059]FIG. 2 is a graph showing the relationship between the amount ofcoordinated water based on the total weight of the polymer electrolytemembrane and ion conductivity;

[0060]FIG. 3 is a graph showing the relationship between the ratio (Y/X)of the content of fluorine in the polymer electrolyte membrane (Y) tothe content of fluorine in the electrode catalyst layer (X) and theoxidation stability of the electrode structure;

[0061]FIG. 4 is a graph showing the relationship between the ratio (Y/X)of the content of fluorine in the polymer electrolyte membrane (Y) tothe content of fluorine in the electrode catalyst layer (X) and thecreep resistance of the electrode structure;

[0062]FIG. 5 is a graph showing a method of examining the powergeneration efficiency of the electrode structure;

[0063]FIG. 6 is a graph showing the power generation efficiency of theelectrode structure;

[0064]FIG. 7 is a graph showing the relationship between the initial ionconductivity of the polymer electrolyte membrane used for the electrodestructure and the amount of a solvent contained in the polymerelectrolyte membrane;

[0065]FIG. 8 is a graph showing the relationship between the ionconductivity retention of the polymer electrolyte membrane used for theelectrode structure and the amount of a solvent contained in the polymerelectrolyte membrane; and

[0066]FIG. 9 is a graph showing the relationship between the toughnessof the polymer electrolyte membrane used for the electrode structure andthe amount of a solvent contained in the polymer electrolyte membrane.

BEST MODE FOR CARRYING OUT THE INVENTION

[0067] First, a first embodiment of the electrode structure of thepresent invention will be explained below.

[0068] As shown in FIG. 1, the electrode structure in the presentembodiment comprises a pair of electrode catalyst layers 1, 1, a polymerelectrolyte membrane 2 sandwiched between both the electrode catalystlayers 1, 1, and backing layers 3, 3 laminated on the electrode catalystlayers 1, 1 respectively.

[0069] The electrode catalyst layer 1 is produced by screen printing acatalyst paste consisting of a catalyst particle and an ion conductingpolymer binder on the backing layer 3, so that a certain amount (e.g.,0.5 mg/cm²) of catalyst is kept thereon, and then drying it. The abovecatalyst particle consists of a platinum particle that is supported bycarbon black (furnace black) at a certain weight ratio (e.g., carbonblack:platinum=1:1). The above catalyst paste is prepared by uniformlydispersing the above catalyst particles in a solution containing an ionconducting polymer binder such as a perfluoroalkylene sulfonic acidpolymer (e.g., Nafion (trade name) from DuPont) at a certain weightratio (e.g., catalyst particle:binder solution=1:1).

[0070] The backing layer 3 consists of a substrate layer and a carbonpaper. The above substrate layer is formed by mixing carbon black andpolytetrafluoroethylene (PTFE) particles at a certain weight ratio(e.g., carbon black:PTFE particle=4:6), uniformly dispersing theobtained mixture in a solvent such as ethylene glycol so as to obtain aslurry, and applying the slurry on the one side of the above carbonpaper followed by drying it.

[0071] The catalyst paste screen printed on the backing layer 3 isdried, for example, by drying at 60° C. for 10 minutes and then vacuumdrying at 120° C. for 60 minutes.

[0072] The polymer electrolyte membrane 2 in the present embodiment is asulfonate of a copolymer obtained by polymerizing a first repeating unitrepresented by the following general formula (1) and a second repeatingunit represented by the following general formula (6) at a certain molarratio, or of a polymer such as polyether ether ketone represented by thefollowing formula (7):

[0073] wherein A represents an electron attracting group, B representsan electron releasing group group, n is an integer of 0 or 1, and abenzene ring includes a derivative thereof, and

[0074] wherein A represents an electron attracting group, Y represents—C(CF₃)₂— or —SO₂—, and a benzene ring includes a derivative thereof, or

[0075] An example of a monomer used as the first repeating unitrepresented by the above general formula (1) includes2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone represented by thefollowing formula (3).

[0076] Examples of a monomer used as the second repeating unitrepresented by the above general formula (6) include2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropanerepresented by the following formula (4) and2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]sulfone represented by thefollowing formula (5):

[0077] The above polymer preferably has a polymer molecular weight of10,000 to 1,000,000 at a weight-average molecular weight shown usingpolystyrene conversion. If the above polymer molecular weight is lessthan 10,000, a mechanical strength that is preferable as a polymerelectrolyte membrane might not be obtained. If it exceeds 1,000,000, asdescribed later, when the polymer is dissolved in a solvent to form amembrane, the dissolubility decreases or the viscosity of the solutionincreases, and thereby it becomes difficult to treat the polymer.

[0078] Thereafter, concentrated sulfuric acid is added to the abovepolymer for sulfonation, such that it contains a sulfonic acid groupwithin the range between 0.5 and 3.0 mg equivalent/g. If the obtainedsulfonate contains less than 0.5 mg equivalent/g of sulfonic acid group,it cannot obtain a sufficient ion conductivity. If the content of asulfonic acid group exceeds 3.0 mg equivalent/g, a sufficient toughnesscannot be obtained, and it makes difficult to treat the sulfonate duringthe production of an electrode structure, which will be described later.

[0079] The sulfonate of the above polymer is then dissolved inN-methylpyrrolidone to prepare a polymer electrolyte solution.Thereafter, a membrane is formed from the polymer electrolyte solutionby the cast method followed by drying in an oven, so as to prepare, forexample, the polymer electrolyte membrane 2 having a dry film thicknessof 50 μm.

[0080] In the present embodiment, the polymer electrolyte membrane 2 issandwiched between the sides of the electrode catalyst layers 1 of theabove electrodes followed by hot pressing, so as to obtain the electrodestructure as shown in FIG. 1. The hot pressing is carried out, forexample, at 150° C. at 2.5 MPa for 1 minute.

[0081] When a separator acting also as a gas passage is furtherlaminated on each of the backing layers 3, 3, the electrode structure inthe present embodiment constitutes a polymer electrolyte fuel cell.

[0082] In the present embodiment, the sulfonate forming the polymerelectrolyte membrane 2 contains at least 5% by weight of the coordinatedwater of a proton of a sulfonic acid group based on the total weight ofthe above polymer electrolyte membrane 2. The coordinated water can bemeasured as follows.

[0083] In the present description, as the above polymer electrolytemembrane 2, a polymer electrolyte membrane comprising a sulfonic acidgroup is referred to as a sulfonic acid type polymer electrolytemembrane. To measure the amount of the coordinated water of the abovepolymer electrolyte membrane 2, first, 50 mg of the sulfonic acid typepolymer electrolyte membrane is left for 1 hour or more under a constanttemperature and constant humidity environment of 85° C. and a relativehumidity of 90%, which simulates the condition of an electrolytemembrane when a fuel cell is in operation, and then the weight of themembrane in a wet state (a) is measured.

[0084] Thereafter, a sample whose membrane weight (a) is measured isdried in a vacuum drying oven at 110° C. for 16 hours, and then theweight of the membrane in a dry state (b) is measured. Now, the contentof water in the sulfonic acid type polymer electrolyte membrane (W₁) isdefined as the amount (a−b) obtained by subtracting the membrane weightin a dry state (b) from the membrane weight in a wet state (a). Thewater content of the sulfonic acid type polymer electrolyte membrane(W₁) is the total amount of free water contained in the electrolytemembrane and the coordinate water.

[0085] Thereafter, 100 mg of the sulfonic acid type polymer electrolytemembrane is immersed in 300 ml of a NaCl aqueous solution (1 mol/l,liquid temperature: 25° C.), so that the proton of the sulfonic acidgroup is substituted by sodium. In the present description, a polymerelectrolyte membrane, in which the proton of the sulfonic acid group issubstituted by sodium, is referred to as a sodium type polymerelectrolyte membrane. Thereafter, 50 mg of the sodium type polymerelectrolyte membrane is treated in the same manner as in the case of thesulfonic acid type polymer electrolyte membrane, and then the wetmembrane weight (c) and the dry membrane weight (d) of the sodium typepolymer electrolyte membrane are measured. Now, the content of water inthe sodium type polymer electrolyte membrane (W₂) is defined as theamount (c−d) obtained by subtracting the dry membrane weight (d) fromthe wet membrane weight (c). In the sodium type polymer electrolytemembrane, the proton of the sulfonic acid group is substituted bysodium, and so no protons are present. Accordingly, the sodium typepolymer electrolyte membrane contains no coordinated water, and itswater content (W₂) indicates the net amount of free water contained inthe electrolyte membrane.

[0086] Thus, the amount of the coordinated water of the polymerelectrolyte membrane 2 (W) can be calculated by obtaining the amount(W₁−W₂) obtained by subtracting the water content of the sodium typepolymer electrolyte membrane (W₂) from the water content of the sulfonicacid type polymer electrolyte membrane (W₁). It should be noted thateach of the water content of the sulfonic acid type polymer electrolytemembrane (W₁) and the water content of the sodium type polymerelectrolyte membrane (W₂) is a mean value obtained from 3 samples.

[0087] Next, the present embodiment will be described in the followingExamples and Comparative Examples.

EXAMPLE 1

[0088] In the present example, first,2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone represented by the aboveformula (3) was polymerized with2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropanerepresented by the above formula (4) at a molar ratio of 90:10, so as toobtain a copolymer (n:m=90:10) represented by formula (8) indicatedbelow.

[0089] Herein, formula (8) indicates a random polymer, which containsthe first repeating unit corresponding to formula (3) and the secondrepeating unit corresponding to formula (4) at a molar ratio of n:m. Itdoes not indicate a block polymer, in which a polymer obtained bybinding n number of the consecutive first repeating units correspondingto formula (3) binds to another polymer obtained by binding m number ofthe consecutive second repeating units corresponding to formula (4).

[0090] Thereafter, concentrated sulfuric acid was added to the abovecopolymer for sulfonation, so as to obtain a sulfonate having an ionexchange capacity of 1.0 meq/g. Thereafter, the sulfonate of the abovecopolymer was dissolved in N-methylpyrrolidone to prepare a polymerelectrolyte solution. A membrane was formed from the polymer electrolytesolution by the cast method followed by drying in an oven, so as toprepare a membrane having a dry film thickness of 50 μm, and themembrane was defined as the polymer electrolyte membrane 2.

[0091] Subsequently, a platinum particle was supported by carbon black(furnace black) at a weight ratio of carbon black platinum=1:1, so as toprepare a catalyst particle. Then, using a solution containing aperfluoroalkylene sulfonic acid polymer (e.g., Nafion (trade name) fromDuPont) as an ion conducting polymer binder, the above catalystparticles were uniformly mixed in the binder at a weight ratio ofbinder:carbon black=1:1, so as to prepare a catalyst paste.

[0092] Thereafter, carbon black was mixed with polytetrafluoroethylene(PTFE) particles at a weight ratio of carbon black:PTFE particle=4:6.The obtained mixture was uniformly dispersed in a solvent such asethylene glycol to obtain a slurry. The obtained slurry was applied onthe one side of the above carbon paper followed by drying it, so as toobtain a substrate layer. Then, two of the backing layers 3 wereprepared, each of which consisted of the substrate layer and the carbonpaper.

[0093] Thereafter, the above catalyst paste was screen printed on eachof the above backing layers 3, so that 0.5 mg/cm² platinum was keptthereon. Then, drying was carried out so as to prepare an electrodecatalyst layer 1. Thus, a pair of electrodes were prepared, each ofwhich consisted of the electrode catalyst layer 1 and the backing layer3.

[0094] Thereafter, the polymer electrolyte membrane 2 was sandwichedbetween the sides of the electrode catalyst layers 1 of the aboveelectrodes, and hot pressing was then carried out to obtain theelectrode structure as shown in FIG. 1.

[0095] Thereafter, the amount of coordinated water contained in thepolymer electrolyte membrane 2 of the present example was measured bythe above described method. Moreover, the polymer electrolyte membrane 2of the present example was fixed in a jig to which a platinum electrodewas attached. The ion conductivity of the membrane was measured by thealternating two-terminal method (frequency: 10 kHz) at −40° C. in a drystate in a low temperature bath. The results are shown in Table 1 andFIG. 2.

EXAMPLE 2

[0096] In the present example, the electrode structure as shown in FIG.1 was obtained completely in the same manner as in Example 1 with theexception that polyether ether ketone represented by the above formula(7) was used instead of the copolymer represented by the above formula(8) in Example 1.

[0097] Thereafter, the amount of coordinated water based on the totalweight of the polymer electrolyte membrane 2 and the ion conductivity ofthe polymer electrolyte membrane 2 in the present example were obtainedcompletely in the same manner as in Example 1. The results are shown inTable 1 and FIG. 2.

EXAMPLE 3

[0098] In the present example, the electrode structure as shown in FIG.1 was obtained completely in the same manner as in Example 1 with theexception that a copolymer (n:m=90:10) represented by formula (9)indicated below was used instead of the copolymer represented by theabove formula (8) in Example 1.

[0099] Herein, formula (9) indicates a random polymer, which containsthe first repeating unit and the second repeating unit at a molar ratioof n:m. It does not indicate a block polymer, in which a polymerobtained by binding n number of the consecutive first repeating unitsbinds to another polymer obtained by binding m number of the consecutivesecond repeating units.

[0100] Thereafter, the amount of coordinated water based on the totalweight of the polymer electrolyte membrane 2 and the ion conductivity ofthe polymer electrolyte membrane 2 in the present example were obtainedcompletely in, the same manner as in Example 1. The results are shown inTable 1 and FIG. 2.

COMPARATIVE EXAMPLE 1

[0101] In the present comparative example, the electrode structure asshown in FIG. 1 was obtained completely in the same manner as in Example1 with the exception that polyether ether ketone type polymerrepresented by the following formula (10) was used instead of thecopolymer represented by the above formula (8) in Example 1:

[0102] Thereafter, the amount of coordinated water based on the totalweight of the polymer electrolyte membrane 2 and the ion conductivity ofthe polymer electrolyte membrane 2 in the present comparative examplewere obtained completely in the same manner as in Example 1. The resultsare shown in Table 1 and FIG. 2.

COMPARATIVE EXAMPLE 2

[0103] In the present comparative example, the electrode structure asshown in FIG. 1 was obtained completely in the same manner as in Example1 with the exception that a perfluoroalkylene sulfonic acid polymer(Nafion 112 (trade name) from DuPont) was used instead of a sulfonate ofthe copolymer represented by the above formula (8) in Example 1.

[0104] Thereafter, the amount of coordinated water based on the totalweight of the polymer electrolyte membrane 2 and the ion conductivity ofthe polymer electrolyte membrane 2 in the present comparative examplewere obtained completely in the same manner as in Example 1. The resultsare shown in Table 1 and FIG. 2. TABLE 1 Amount of coordinated Ionconductivity water (weight %) (S/cm) Example 1 6.4 0.0071 Example 2 5.50.0054 Example 3 10.0 0.0080 Comparative 0.7 0.0004 Example 1Comparative 3.6 0.0023 Example 2

[0105] From Table 1 and FIG. 2, it is clear that when compared with theelectrode structures of Comparative Examples 1 and 2 in which the amountof coordinated water based on the total weight of the polymerelectrolyte membrane 2 is less than 5% by weight, the electrodestructures of Examples 1 to 3 in which the above amount is 5 or more %by weight have an excellent ion conductivity even in a low temperaturecondition of −40° C.

[0106] Next, a second embodiment of the electrode structure of thepresent invention will be explained below.

[0107] As shown in FIG. 1, the electrode structure in the presentembodiment comprises a pair of electrode catalyst layers 1, 1, a polymerelectrolyte membrane 2 sandwiched between both the electrode catalystlayers 1, 1, and backing layers 3, 3 laminated on the electrode catalystlayers 1, 1 respectively.

[0108] The electrode catalyst layer 1 is produced by screen printing acatalyst paste consisting of a catalyst particle and afluorine-containing ion conducting polymer binder on the backing layer3, so that a certain amount (e.g., 0.5 mg/cm²) of catalyst is keptthereon, and then drying it. The above catalyst particle consists of aplatinum particle that is supported by carbon black (furnace black) at acertain weight ratio (e.g., carbon black:platinum=1:1). The abovecatalyst paste is prepared by uniformly dispersing the above catalystparticles in a solution containing a fluorine-containing ion conductingpolymer binder such as a perfluoroalkylene sulfonic acid polymer (e.g.,Nafion (trade name) from DuPont) at a certain weight ratio (e.g.,catalyst particle:binder solution=1:1).

[0109] The backing layer 3 consists of a substrate layer and a carbonpaper. The above substrate layer is formed by mixing carbon black andpolytetrafluoroethylene (PTFE) particles at a certain weight ratio(e.g., carbon black:PTFE particle=4:6), uniformly dispersing theobtained mixture in a solvent such as ethylene glycol so as to obtain aslurry, and applying the slurry on the one side of the above carbonpaper followed by drying it.

[0110] The catalyst paste screen printed on the backing layer 3 isdried, for example, by drying at 60° C. for 10 minutes and then vacuumdrying at 120° C. for 60 minutes.

[0111] The polymer electrolyte membrane 2 is a sulfonate of afluorine-containing copolymer represented by the following formula (8)that is obtained, for example, by polymerizing2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone represented by thefollowing formula (3) with2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropanerepresented by the following formula (4) at a certain polymerizationratio:

[0112] The above fluorine-containing copolymer preferably has a polymermolecular weight of 10,000 to 1,000,000 at a weight-average molecularweight shown using polystyrene conversion. If the above polymermolecular weight is less than 10,000, a mechanical strength that ispreferable as a polymer electrolyte membrane might not be obtained. Ifit exceeds 1,000,000, as described later, when the polymer is dissolvedin a solvent to form a membrane, the dissolubility decreases or theviscosity of the solution increases, and thereby it becomes difficult totreat the polymer.

[0113] Thereafter, concentrated sulfuric acid is added to the abovecopolymer for sulfonation, such that the sulfonate of the abovefluorine-containing copolymer contains sulfonic acid groups within therange between 0.5 and 3.0 mg equivalent/g. If the obtained sulfonatecontains less than 0.5 mg equivalent/g of sulfonic acid group, it cannotobtain a sufficient ion conductivity. If the content of a sulfonic acidgroup exceeds 3.0 mg equivalent/g, a sufficient toughness cannot beobtained, and it makes difficult to treat the sulfonate during theproduction of an electrode structure, which will be described later.

[0114] The sulfonate of the above fluorine-containing copolymer is thendissolved in N-methylpyrrolidone to prepare a polymer electrolytesolution. Thereafter, a membrane is formed from the polymer electrolytesolution by the cast method followed by drying in an oven, so as toprepare, for example, the polymer electrolyte membrane 2 having a dryfilm thickness of 50 μm. Alternatively, the polymer electrolyte membrane2 may also be prepared as a composite membrane. The composite membranecomprises a fluorine-containing ion conducting polymer-coated layer witha dry film thickness of 5 μm, for example, which is formed by furthercasting a fluorine-containing ion conducting polymer solution such as aperfluoroalkylene sulfonic acid polymer (e.g., Nafion (trade name) fromDuPont) on both sides of the membrane formed from the above polymerelectrolyte solution.

[0115] In the present embodiment, the polymer electrolyte membrane 2 issandwiched between the sides of the electrode catalyst layers 1 of theabove electrodes followed by hot pressing, so as to obtain the electrodestructure as shown in FIG. 1. The hot pressing is carried out by, forexample, performing the first pressing at 80° C. at 5 MPa for 2 minutesand then the second pressing at 160° C. at 4 MPa for 1 minute.

[0116] When a separator acting also as a gas passage is furtherlaminated on each of the backing layers 3, 3, the electrode structure inthe present embodiment constitutes a polymer electrolyte fuel cell.

[0117] Next, the present embodiment will be described in the followingexamples and comparative examples.

EXAMPLE 4

[0118] In the present example, first,2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone represented by formula(3) was polymerized with2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropanerepresented by formula (4) at a molar ratio of 88:12, so as to obtain afluorine-containing copolymer (n:m=88:12) represented by formula (8).

[0119] Thereafter, concentrated sulfuric acid was added to the abovecopolymer for sulfonation, so as to obtain a sulfonate having an ionexchange capacity of 1.9 meq/g. Thereafter, the sulfonate of the abovecopolymer was dissolved in N-methylpyrrolidone to prepare a polymerelectrolyte solution. A membrane was formed from the polymer electrolytesolution by the cast method followed by drying in an oven, so as toprepare the polymer electrolyte membrane 2 having a dry film thicknessof 50 μm. The content of fluorine in the polymer electrolyte membrane 2(Y) was 10% by weight.

[0120] Subsequently, a platinum particle was supported by carbon black(furnace black) at a weight ratio of carbon black:platinum=1:1, so as toprepare a catalyst particle. Then, using a solution containing aperfluoroalkylene sulfonic acid polymer (Nafion (trade name) fromDuPont) as a fluorine-containing ion conducting polymer binder, theabove catalyst particles were uniformly dispersed in the binder at aweight ratio of binder:carbon black=1:1, so as to prepare a catalystpaste.

[0121] Thereafter, carbon black was mixed with polytetrafluoroethylene(PTFE) particles at a weight ratio of carbon black:PTFE particle=4:6.The obtained mixture was uniformly dispersed in a solvent such asethylene glycol to obtain a slurry. The obtained slurry was applied onthe one side of the above carbon paper followed by drying it, so as toobtain a substrate layer. Then, two of the backing layers 3 wereprepared, each of which consisted of the substrate layer and the carbonpaper.

[0122] Thereafter, the above catalyst paste was screen printed on eachof the above backing layers 3, so that 0.5 mg/cm² platinum was keptthereon. Then, drying was carried out so as to prepare an electrodecatalyst layer 1. Thus, a pair of electrodes were prepared, each ofwhich consisted of the electrode catalyst layer 1 and the backing layer3. The content of fluorine in the electrode catalyst layer 1 (X) was 24%by weight.

[0123] Thereafter, the polymer electrolyte membrane 2 was sandwichedbetween the sides of the electrode catalyst layers 1 of the aboveelectrodes, and hot pressing was then carried out to obtain theelectrode structure as shown in FIG. 1. As a result, in the electrodestructure in the present example, the ratio (Y/X) of the content offluorine in the polymer electrolyte membrane 2 (Y) to the content offluorine in the electrode catalyst layer 1 (X) was 0.42.

[0124] Thereafter, the oxidation stability, creep resistance, and powergeneration efficiency of the electrode structure in the present examplewere evaluated. The polymer electrolyte membrane 2 was immersed for 10hours in an aqueous solution (Fenton's reagent) with a H₂O₂concentration of 3%, a Fe concentration of 20 ppm, and a liquidtemperature of 40° C., and then its weight reduction rate (%) wasmeasured. The oxidation stability was defined as such a weight reductionrate. The above weight reduction rate indicates the amount of thepolymer electrolyte membrane 2 dissolved in the above reagent. Thesmaller the figure, the higher the oxidation stability that can beobtained. The results are shown in Table 2. The relationship between theratio (Y/X) and oxidation stability is shown in FIG. 3.

[0125] A load was applied to the above electrode structure at a pressureof 5 kg/cm² for 1,000 hours under the environment of a temperature of90° C. and a relative humidity of 90%, and then the thickness reductionrate (%) of the electrode structure was measured. The creep resistancewas defined as such a thickness reduction rate. The smaller thethickness reduction rate, the higher the creep resistance that can beobtained. The results are shown in Table 2. The relationship between theratio (Y/X) and creep resistance is shown in FIG. 4.

[0126] The power generation efficiency was evaluated as follows. Theabove electrode structure was used for a single cell. Air was suppliedto one backing layer 3 as an oxygen electrode, whereas pure hydrogen wassupplied to the other backing layer 3 as a fuel electrode, so as togenerate electric power. Power generation conditions were a temperatureof 90° C., a relative humidity of 50% on the fuel electrode side, and arelative humidity of 80% on the oxygen electrode side. As shown in FIG.5, as current density increased, cell voltage gradually decreased. Thus,cell voltage was measured at a current density of 0.5 A/cm². If themeasured cell voltage was 0.4 V or greater, it was evaluated that thecell had a good power generation efficiency. The results are shown inTable 2.

EXAMPLE 5

[0127] In the present example, the electrode structure as shown in FIG.1 was produced completely in the same manner as in Example 4 with theexception that 2,5-dichloro-4′-(4-phenoxyphenoxy) benzophenonerepresented by formula (3) was polymerized with2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropanerepresented by formula (4) at a molar ratio of 74:26, so as to obtain afluorine-containing copolymer (n:m=74:26) represented by formula (8).

[0128] In the electrode structure in the present example, the content offluorine in the polymer electrolyte membrane 2 (Y) was 20% by weight,the content of fluorine in the electrode catalyst layer 1 (X) was 24% byweight, and the ratio (Y/X) of the content of fluorine in the polymerelectrolyte membrane 2 (Y) to the content of fluorine in the electrodecatalyst layer 1 (X) was 0.83.

[0129] Thereafter, the oxidation stability, creep resistance, and powergeneration efficiency of the electrode structure in the present examplewere evaluated in the same manner as in Example 4. The results are shownin Table 2 and FIGS. 3 and 4.

EXAMPLE 6

[0130] In the present example, the electrode structure as shown in FIG.1 was produced completely in the same manner as in Example 4 with theexception that 2,5-dichloro-4′-(4-phenoxyphenoxy) benzophenonerepresented by formula (3) was polymerized with2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropanerepresented by formula (4) at a molar ratio of 74:26 to obtain afluorine-containing copolymer (n:m=74:26) represented by formula (8),and that the weight ratio of an ion conducting polymer binder and carbonblack in the above catalyst paste forming the electrode catalyst layer 1was set at binder:carbon black=1:2.

[0131] In the electrode structure in the present example, the content offluorine in the polymer electrolyte membrane 2 (Y) was 20% by weight,the content of fluorine in the electrode catalyst layer 1 (X) was 15% byweight, and the ratio (Y/X) of the content of fluorine in the polymerelectrolyte membrane 2 (Y) to the content of fluorine in the electrodecatalyst layer 1 (X) was 1.33.

[0132] Thereafter, the oxidation stability, creep resistance, and powergeneration efficiency of the electrode structure in the present examplewere evaluated in the same manner as in Example 4. The results are shownin Table 2 and FIGS. 3 and 4.

EXAMPLE 7

[0133] In the present example, the electrode structure as shown in FIG.1 was produced completely in the same manner as in Example 4 with theexception that 2,5-dichloro-4′-(4-phenoxyphenoxy) benzophenonerepresented by formula (3) was polymerized with2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropanerepresented by formula (4) at a molar ratio of 74:26 to obtain afluorine-containing copolymer (n:m=74:26) represented by formula (8),and that the weight ratio of an ion conducting polymer binder and carbonblack in the above catalyst paste forming the electrode catalyst layer 1was set at binder:carbon black=7:4.

[0134] In the electrode structure in the present example, the content offluorine in the polymer electrolyte membrane 2 (Y) was 20% by weight,the content of fluorine in the electrode catalyst layer 1 (X) was 35% byweight, and the ratio (Y/X) of the content of fluorine in the polymerelectrolyte membrane 2 (Y) to the content of fluorine in the electrodecatalyst layer 1 (X) was 0.57.

[0135] Thereafter, the oxidation stability, creep resistance, and powergeneration efficiency of the electrode structure in the present examplewere evaluated in the same manner as in Example 4. The results are shownin Table 2 and FIGS. 3 and 4.

EXAMPLE 8

[0136] In the present example, the electrode structure as shown in FIG.1 was produced completely in the same manner as in Example 4 with theexception that 2,5-dichloro-4′-(4-phenoxyphenoxy) benzophenonerepresented by formula (3) was polymerized with2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropanerepresented by formula (4) at a molar ratio of 92:8 to obtain afluorine-containing copolymer (n:m=92:8) represented by formula (8).

[0137] In the electrode structure in the present example, the content offluorine in the polymer electrolyte membrane 2 (Y) was 7% by weight, thecontent of fluorine in the electrode catalyst layer 1 (X) was 24% byweight, and the ratio (Y/X) of the content of fluorine in the polymerelectrolyte membrane 2 (Y) to the content of fluorine in the electrodecatalyst layer 1 (X) was 0.29.

[0138] Thereafter, the oxidation stability, creep resistance, and powergeneration efficiency of the electrode structure in the present examplewere evaluated in the same manner as in Example 4. The results are shownin Table 2 and FIGS. 3 and 4.

EXAMPLE 9

[0139] In the present example, the electrode structure as shown in FIG.1 was produced completely in the same manner as in Example 4 with theexception that the weight ratio of an ion conducting polymer binder andcarbon black in the above catalyst paste forming the electrode catalystlayer 1 was set at binder:carbon black=1:2.

[0140] In the electrode structure in the present example, the content offluorine in the polymer electrolyte membrane 2 (Y) was 10% by weight,the content of fluorine in the electrode catalyst layer 1 (X) was 35% byweight, and the ratio (Y/X) of the content of fluorine in the polymerelectrolyte membrane 2 (Y) to the content of fluorine in the electrodecatalyst layer 1 (X) was 0.67.

[0141] Thereafter, the oxidation stability, creep resistance, and powergeneration efficiency of the electrode structure in the present examplewere evaluated in the same manner as in Example 4. The results are shownin Table 2 and FIGS. 3 and 4.

EXAMPLE 10

[0142] In the present example, the electrode structure as shown in FIG.1 was produced completely in the same manner as in Example 4 with theexception that the weight ratio of an ion conducting polymer binder andcarbon black in the above catalyst paste forming the electrode catalystlayer 1 was set at binder:carbon black=7:4.

[0143] In the electrode structure in the present example, the content offluorine in the polymer electrolyte membrane 2 (Y) was 20% by weight,the content of fluorine in the electrode catalyst layer 1 (X) was 35% byweight, and the ratio (Y/X) of the content of fluorine in the polymerelectrolyte membrane 2 (Y) to the content of fluorine in the electrodecatalyst layer 1 (X) was 0.29.

[0144] Thereafter, the oxidation stability, creep resistance, and powergeneration efficiency of the electrode structure in the present examplewere evaluated in the same manner as in Example 4. The results are shownin Table 2 and FIGS. 3 and 4.

EXAMPLE 11

[0145] In the present example,2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone represented by formula(3) was polymerized with2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropanerepresented by formula (4) at a molar ratio of 88:12, so as to obtain afluorine-containing copolymer (n:m=88:12) represented by formula (8).Then, a solution containing a perfluoroalkylene sulfonic acid polymer(Nafion (trade name) from DuPont) was casted on both sides of a membraneformed of the sulfonate of the above obtained copolymer, so as to obtaina composite membrane in which a fluorine-containing ion conductingpolymer-coated layer with a dry film thickness of 5 μm was formed. Thiscomposite membrane was defined as the polymer electrolyte membrane 2.Moreover, the weight ratio of an ion conducting polymer binder andcarbon black in the above catalyst paste forming the electrode catalystlayer 1 was set at binder:carbon black=7:4. Other than the aboveexceptions, the electrode structure as shown in FIG. 1 was producedcompletely in the same manner as in Example 4.

[0146] In the electrode structure in the present example, the content offluorine in the polymer electrolyte membrane 2 (Y) was 22% by weight,the content of fluorine in the electrode catalyst layer 1 (X) was 35% byweight, and the ratio (Y/X) of the content of fluorine in the polymerelectrolyte membrane 2 (Y) to the content of fluorine in the electrodecatalyst layer 1 (X) was 0.63.

[0147] Thereafter, the oxidation stability, creep resistance, and powergeneration efficiency of the electrode structure in the present examplewere evaluated in the same manner as in Example 4. The results are shownin Table 2 and FIGS. 3 and 4.

EXAMPLE 12

[0148] In the present example, 2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone represented by formula (3) was polymerized with2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropanerepresented by formula (4) at a molar ratio of 74:26, so as to obtain afluorine-containing copolymer (n:m=74:26) represented by formula (8).Then, a solution containing a perfluoroalkylene sulfonic acid polymer(Nafion (trade name) from DuPont) was casted on both sides of a membraneformed of the sulfonate of the above obtained copolymer, so as to obtaina composite membrane in which a fluorine-containing ion conductingpolymer-coated layer with a dry film thickness of 5 μm was formed. Thiscomposite membrane was defined as the polymer electrolyte membrane 2.Other than the above exceptions, the electrode structure as shown inFIG. 1 was produced completely in the same manner as in Example 4.

[0149] In the electrode structure in the present example, the content offluorine in the polymer electrolyte membrane 2 (Y) was 40% by weight,the content of fluorine in the electrode catalyst layer 1 (X) was 24% byweight, and the ratio (Y/X) of the content of fluorine in the polymerelectrolyte membrane 2 (Y) to the content of fluorine in the electrodecatalyst layer 1 (X) was 1.67.

[0150] Thereafter, the oxidation stability, creep resistance, and powergeneration efficiency of the electrode structure in the present examplewere evaluated in the same manner as in Example 4. The results are shownin Table 2 and FIGS. 3 and 4.

COMPARATIVE EXAMPLE 3

[0151] In the present comparative example, the electrode structure asshown in FIG. 1 was produced completely in the same manner as in Example4 with the exception that a membrane consisting of a perfluoroalkylenesulfonic acid polymer (Nafion 112 (trade name) from DuPont) was used asthe polymer electrolyte membrane 2.

[0152] In the electrode structure in the present comparative example,the content of fluorine in the polymer electrolyte membrane 2 (Y) was73% by weight, the content of fluorine in the electrode catalyst layer 1(X) was 24% by weight, and the ratio (Y/X) of the content of fluorine inthe polymer electrolyte membrane 2 (Y) to the content of fluorine in theelectrode catalyst layer 1 (X) was 3.04.

[0153] Thereafter, the oxidation stability, creep resistance, and powergeneration efficiency of the electrode structure in the presentcomparative example were evaluated in the same manner as in Example 4.The results are shown in Table 2 and FIGS. 3 and 4.

COMPARATIVE EXAMPLE 4

[0154] In the present comparative example, the electrode structure asshown in FIG. 1 was produced completely in the same manner as in Example4 with the exception that a membrane consisting of a perfluoroalkylenesulfonic acid polymer (Nafion 112 (trade name) from DuPont) was used asthe polymer electrolyte membrane 2, and that the weight ratio of an ionconducting polymer binder and carbon black in the above catalyst pasteforming the electrode catalyst layer 1 was set at binder:carbonblack=1:2.

[0155] In the electrode structure in the present comparative example,the content of fluorine in the polymer electrolyte membrane 2 (Y) was73% by weight, the content of fluorine in the electrode catalyst layer 1(X) was 15% by weight, and the ratio (Y/X) of the content of fluorine inthe polymer electrolyte membrane 2 (Y) to the content of fluorine in theelectrode catalyst layer 1 (X) was 4.87.

[0156] Thereafter, the oxidation stability, creep resistance, and powergeneration efficiency of the electrode structure in the presentcomparative example were evaluated in the same manner as in Example 4.The results are shown in Table 2 and FIGS. 3 and 4.

COMPARATIVE EXAMPLE 5

[0157] In the present comparative example, the electrode structure asshown in FIG. 1 was produced completely in the same manner as in Example4 with the exception that a membrane consisting of a perfluoroalkylenesulfonic acid polymer (Nafion 112 (trade name) from DuPont) was used asthe polymer electrolyte membrane 2, and that the weight ratio of an ionconducting polymer binder and carbon black in the above catalyst pasteforming the electrode catalyst layer 1 was set at binder:carbonblack=7:4.

[0158] In the electrode structure in the present comparative example,the content of fluorine in the polymer electrolyte membrane 2 (Y) was73% by weight, the content of fluorine in the electrode catalyst layer 1(X) was 35% by weight, and the ratio (Y/X) of the content of fluorine inthe polymer electrolyte membrane 2 (Y) to the content of fluorine in theelectrode catalyst layer 1 (X) was 2.09.

[0159] Thereafter, the oxidation stability, creep resistance, and powergeneration efficiency of the electrode structure in the presentcomparative example were evaluated in the same manner as in Example 4.The results are shown in Table 2 and FIGS. 3 and 4.

COMPARATIVE EXAMPLE 6

[0160] In the present comparative example,2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone represented by formula(3) was polymerized with2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropanerepresented by formula (4) at a molar ratio of 88:12, so as to obtain afluorine-containing copolymer (n:m=88:12) represented by formula (8).Then, a solution containing a perfluoroalkylene sulfonic acid polymer(Nafion (trade name) from DuPont) was casted on both sides of a membraneformed of the sulfonate of the above obtained copolymer, so as to obtaina composite membrane in which a fluorine-containing ion conductingpolymer-coated layer with a dry film thickness of 15 μm was formed. Thiscomposite membrane was defined as the polymer electrolyte membrane 2.Other than the above exceptions, the electrode structure as shown inFIG. 1 was produced completely in the same manner as in Example 4.

[0161] In the electrode structure in the present comparative example,the content of fluorine in the polymer electrolyte membrane 2 (Y) was50% by weight, the content of fluorine in the electrode catalyst layer 1(X) was 24% by weight, and the ratio (Y/X) of the content of fluorine inthe polymer electrolyte membrane 2 (Y) to the content of fluorine in theelectrode catalyst layer 1 (X) was 2.08.

[0162] Thereafter, the oxidation stability, creep resistance, and powergeneration efficiency of the electrode structure in the presentcomparative example were evaluated in the same manner as in Example 4.The results are shown in Table 2 and FIGS. 3 and 4.

COMPARATIVE EXAMPLE 7

[0163] In the present comparative example, the electrode structure asshown in FIG. 1 was produced completely in the same manner as in Example4 with the exception that 2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone represented by formula (3) was polymerized with2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropanerepresented by formula (4) at a molar ratio of 97:3, so as to obtain afluorine-containing copolymer (n:m=97:3) represented by formula (8).

[0164] In the electrode structure in the present comparative example,the content of fluorine in the polymer electrolyte membrane 2 (Y) was 3%by weight, the content of fluorine in the electrode catalyst layer 1 (X)was 24% by weight, and the ratio (Y/X) of the content of fluorine in thepolymer electrolyte membrane 2 (Y) to the content of fluorine in theelectrode catalyst layer 1 (X) was 0.13.

[0165] Thereafter, the oxidation stability, creep resistance, and powergeneration efficiency of the electrode structure in the presentcomparative example were evaluated in the same manner as in Example 4.The results are shown in Table 2 and FIGS. 3 and 4. TABLE 2 Creepresistance Oxidation Power generation Y/X (%) stability (%) efficiencyExample 4 0.42 −3 −15 G Example 5 0.83 −3 −12 G Example 6 1.33 −3 −10 GExample 7 0.57 −5 −15 G Example 8 0.29 −2 −19 G Example 9 0.67 −2 −15 GExample 10 0.29 −4 −18 G Example 11 0.63 −4 −15 G Example 12 1.67 −10 −5G Comparative 3.04 −40 0 G Example 3 Comparative 4.87 −42 0 G Example 4Comparative 2.09 −39 0 G Example 5 Comparative 2.08 −25 −5 G Example 6Comparative 0.13 −2 −45 G Example 7

[0166] As is clear from the results shown in Table 2 and FIGS. 3 and 4,the electrode structures of Examples 4 to 12 in which the ratio (Y/X) ofthe content of fluorine in the polymer electrolyte membrane 2 (Y) to thecontent of fluorine in the electrode catalyst layer 1 (X) is within therange between 0.29 and 1.67 are excellent in both creep resistance andoxidation stability. In addition, they are good also in power generationefficiency.

[0167] In contrast, it is clear that the electrode structures ofComparative Examples 3 to 6 in which the ratio (Y/X) of the content offluorine in the polymer electrolyte membrane 2 (Y) to the content offluorine in the electrode catalyst layer 1 (X) is more than 2.0 areexcellent in oxidation stability, but they are poor in creep resistance.Moreover, it is also clear that the electrode structure of ComparativeExample 7 in which the above ratio (Y/X) is less than 0.2 is excellentin creep resistance, but it is poor in oxidation stability.

[0168] Next, a third embodiment of the electrode structure of thepresent invention will be explained below.

[0169] As shown in FIG. 1, the electrode structure in the presentembodiment comprises a pair of electrode catalyst layers 1, 1, a polymerelectrolyte membrane 2 sandwiched between both the electrode catalystlayers 1, 1, and backing layers 3, 3 laminated on the electrode catalystlayers 1, 1 respectively.

[0170] The electrode catalyst layer 1 contains platinum as a catalyst,and it is produced by screen printing a catalyst paste consisting of acatalyst particle and an ion conducting polymer binder on the backinglayer 3, so that the content of platinum on the layer is within therange between 0.01 and 0.8 mg/cm², and then drying it. The abovecatalyst particle consists of a platinum particle that is supported by acarbon black (furnace black) particle having an average particle size of10 to 100 nm at a certain weight ratio (e.g., carbonblack:platinum=1:1). The above catalyst paste is prepared by uniformlydispersing the above catalyst particles in a solution containing an ionconducting polymer binder such as a perfluoroalkylene sulfonic acidpolymer (e. g., Nafion (trade name) from DuPont) at a certain weightratio (e.g., catalyst particle:binder solution=1:1).

[0171] The backing layer 3 consists of a substrate layer and a carbonpaper. The above substrate layer is formed by mixing carbon black andpolytetrafluoroethylene (PTFE) particles at a certain weight ratio(e.g., carbon black:PTFE particle=4:6), uniformly dispersing theobtained mixture in a solvent such as ethylene glycol so as to obtain aslurry, and applying the slurry on the one side of the above carbonpaper followed by drying it. The catalyst paste screen printed on thebacking layer 3 is dried, for example, by drying at 60° C. for 10minutes and then vacuum drying at 120° C. for 60 minutes.

[0172] The polymer electrolyte membrane 2 in the present embodiment is acopolymer obtained by polymerizing a first repeating unit represented bythe following general formula (1) and a second repeating unitrepresented by the following general formula (2) at a certain molarratio:

[0173] wherein A represents an electron attracting group, B representsan electron releasing group group, n is an integer of 0 or 1, and abenzene ring includes a derivative thereof, and

[0174] wherein A represents an electron attracting group, B representsan electron releasing group group, Y represents —C(CF₃)₂— or —SO₂—, anda benzene ring includes a derivative thereof.

[0175] An example of a monomer used as the first repeating unitrepresented by the above general formula (1) includes2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone represented by thefollowing formula (3).

[0176] Examples of a monomer used as the second repeating unitrepresented by the above general formula (2) include2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropanerepresented by the following formula (4) and2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]sulfone represented by thefollowing formula (5):

[0177] The above polymer preferably has a polymer molecular weight of10,000 to 1,000,000 at a weight-average molecular weight shown usingpolystyrene conversion. If the above polymer molecular weight is lessthan 10,000, a mechanical strength that is preferable as a polymerelectrolyte membrane might not be obtained. If it exceeds 1,000,000, asdescribed later, when the polymer is dissolved in a solvent to form amembrane, the dissolubility decreases or the viscosity of the solutionincreases, and thereby it becomes difficult to treat the polymer.

[0178] Thereafter, concentrated sulfuric acid is added to the abovepolymer for sulfonation, such that it contains a sulfonic acid groupwithin the range between 0.5 and 3.0 mg equivalent/g. If the obtainedsulfonate contains less than 0.5 mg equivalent/g of sulfonic acid group,it cannot obtain a sufficient ion conductivity. If the content of asulfonic acid group exceeds 3.0 mg equivalent/g, a sufficient toughnesscannot be obtained, and it makes difficult to treat the sulfonate duringthe production of an electrode structure, which will be described later.

[0179] The sulfonate of the above polymer is then dissolved inN-methylpyrrolidone to prepare a polymer electrolyte solution.Thereafter, a membrane is formed from the polymer electrolyte solutionby the cast method followed by drying in an oven, so as to prepare, forexample, the polymer electrolyte membrane 2 having a dry film thicknessof 50 μm.

[0180] In the present embodiment, the polymer electrolyte membrane 2 issandwiched between the sides of the electrode catalyst layers 1 of theabove electrodes followed by hot pressing, so as to obtain the electrodestructure as shown in FIG. 1. The hot pressing is carried out, forexample, at 150° C. at 2.5 MPa for 1 minute.

[0181] When a separator acting also as a gas passage is furtherlaminated on each of the backing layers 3, 3, the electrode structure inthe present embodiment constitutes a polymer electrolyte fuel cell.

[0182] Next, the present embodiment will be described in the followingexamples and comparative examples.

EXAMPLE 13

[0183] In the present example, first,2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone represented by the aboveformula (3) was polymerized with2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropanerepresented by the above formula (4) at a polymerization ratio of 50:50,so as to obtain a copolymer (n:m=50:50) represented by the followingformula (8):

[0184] Thereafter, concentrated sulfuric acid was added to the abovecopolymer for sulfonation, so as to obtain a sulfonate having an ionexchange capacity of 2.1 meq/g. Thereafter, the sulfonate of the abovecopolymer was dissolved in N-methylpyrrolidone to prepare a polymerelectrolyte solution, and the polymer electrolyte membrane 2 having adry film thickness of 50 μm was prepared from the polymer electrolytesolution by the cast method.

[0185] Subsequently, a platinum particle was supported by carbon black(furnace black) having an average diameter of 50 nm at a weight ratio ofcarbon black:platinum=1:1, so as to prepare a catalyst particle. Then,the above catalyst particles were uniformly dispersed in a solutioncontaining a perfluoroalkylene sulfonic acid polymer (Nafion (tradename) from DuPont) as an ion conducting binder at a weight ratio of ionconducting binder:catalyst particle=8:5, so as to prepare a catalystpaste.

[0186] Thereafter, carbon black was mixed with polytetrafluoroethylene(PTFE) particles at a weight ratio of carbon black:PTFE particle=4:6.The obtained mixture was uniformly dispersed in ethylene glycol toobtain a slurry. The obtained slurry was applied on the one side of theabove carbon paper followed by drying it, so as to obtain a substratelayer. Then, two of the backing layers 3 were prepared, each of whichconsisted of the substrate layer and the carbon paper.

[0187] Thereafter, the above catalyst paste was screen printed on eachof the above backing layers 3, so that 0.5 mg/cm² platinum was keptthereon. Then, drying was carried out so as to prepare an electrodecatalyst layer 1. Thus, a pair of electrodes were prepared, each ofwhich consisted of the electrode catalyst layer 1 and the backing layer3. The above drying was carried out by drying at 60° C. for 10 minutesand then vacuum drying at 120° C. for 60 minutes.

[0188] Thereafter, the polymer electrolyte membrane 2 was sandwichedbetween the sides of the electrode catalyst layers 1 of the aboveelectrodes, and hot pressing was then carried out to obtain theelectrode structure as shown in FIG. 1. The hot pressing was carried outby performing the first pressing at 80° C. at 5 MPa for 2 minutes andthen the second pressing at 160° C. at 4 MPa for 1 minute.

[0189] The polymer electrolyte membrane 2 used in the present examplehad an excellent toughness, and so the process of sandwiching themembrane between the above pair of electrodes and performing hotpressing thereon was carried out easily.

[0190] Thereafter, the electrode structure obtained in the presentexample was used for a single cell, and its power generation efficiencywas examined. Air was supplied to one backing layer 3 as an oxygenelectrode, whereas pure hydrogen was supplied to the other backing layer3 as a fuel electrode, so as to generate electric power. Electric powerwas generated at a current density of 1 A/cm² for 200 hours, andthereafter, cell potential was measured at a current density of 1 A/cm².Power generation conditions were a temperature of 85° C., a relativehumidity of 40% on the fuel electrode side, and a relative humidity of75% on the oxygen electrode side.

[0191] As a result, the cell potential of the electrode structure in thepresent example was 0.62 V. The results are shown in FIG. 6.

EXAMPLE 14

[0192] In the present example, the electrode structure as shown in FIG.1 was produced completely in the same manner as in Example 13 with theexception that 2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]sulfonerepresented by the above formula (5) was used instead of2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropanerepresented by the above formula (4). Then, the obtained electrodestructure was used for a single cell, and the power generationefficiency was examined completely in the same manner as in Example 13.As a result, the cell potential of the electrode structure in thepresent example was 0.63 V. The results are shown in FIG. 6.

COMPARATIVE EXAMPLE 8

[0193] In the present comparative example, the electrode structure asshown in FIG. 1 was produced completely in the same manner as in Example13 with the exception that the polymer electrolyte membrane 2 comprisingpolyether ether ketone represented by formula (7) indicated below wasused. Then, the obtained electrode structure was used for a single cell,and the power generation efficiency was examined completely in the samemanner as in Example 13. As a result, the cell potential of theelectrode structure in the present comparative example was 0.52 V. Theresults are shown in FIG. 6.

COMPARATIVE EXAMPLE 9

[0194] In the present comparative example, the electrode structure asshown in FIG. 1 was produced completely in the same manner as in Example13 with the exception that the polymer electrolyte membrane 2 comprisingpolybenzimidazole was used. Then, the obtained electrode structure wasused for a single cell, and the power generation efficiency was examinedcompletely in the same manner as in Example 13. As a result, the cellpotential of the electrode structure in the present comparative examplewas 0.50 V. The results are shown in FIG. 6.

[0195] As is clear from FIG. 6, the electrode structures of Examples 13and 14 have a power generation efficiency much more excellent than theelectrode structure (Comparative Example 8) in which the polymerelectrolyte membrane 2 comprising polyether ether ketone was used, orthan the electrode structure (Comparative Example 9) in which thepolymer electrolyte membrane 2 comprising polybenzimidazole was used.

[0196] Next, a fourth embodiment of the electrode structure of thepresent invention will be explained below.

[0197] As shown in FIG. 1, the electrode structure in the presentembodiment comprises a pair of electrode catalyst layers 1, 1, a polymerelectrolyte membrane 2 sandwiched between both the electrode catalystlayers 1, 1, and backing layers 3, 3 laminated on the electrode catalystlayers 1, 1 respectively.

[0198] The electrode catalyst layer 1 is formed by screen printing acatalyst paste consisting of a catalyst particle and an ion conductingpolymer binder on the backing layer 3, so that a certain amount ofcatalyst (e.g., 0.5 mg/cm²) is kept thereon, and then drying it. Theabove catalyst particle consists of a platinum particle that issupported by carbon black (furnace black) at a certain weight ratio(e.g., carbon black:platinum=1:1). The above catalyst paste is preparedby uniformly dispersing the above catalyst particles in an ionconducting polymer binder solution such as a perfluoroalkylene sulfonicacid polymer (e.g., Nafion (trade name) from DuPont) at a certain weightratio (e.g., catalyst particle:binder solution=1:1).

[0199] The backing layer 3 consists of a substrate layer and a carbonpaper. The above substrate layer is formed by mixing carbon black andpolytetrafluoroethylene (PTFE) particles at a certain weight ratio(e.g., carbon black:PTFE particle=4:6), uniformly dispersing theobtained mixture in a solvent such as ethylene glycol so as to obtain aslurry, and applying the slurry on the one side of the above carbonpaper followed by drying it. The catalyst paste screen printed on thebacking layer 3 is dried, for example, by drying at 60° C. for 10minutes and then vacuum drying at 120° C. for 60 minutes.

[0200] The polymer electrolyte membrane 2 in the present embodiment is acopolymer obtained by polymerizing a first repeating unit represented bythe following general formula (1) and a second repeating unitrepresented by the following general formula (2) at a certain molarratio:

[0201] wherein A represents an electron attracting group, B representsan electron releasing group group, n is an integer of 0 or 1, and abenzene ring includes a derivative thereof, and

[0202] wherein A represents an electron attracting group, B representsan electron releasing group group, Y represents —C(CF₃)₂— or —SO₂—, anda benzene ring includes a derivative thereof.

[0203] An example of a monomer used as the first repeating unitrepresented by the above general formula (1) includes2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone represented by formula(3) indicated below.

[0204] Examples of a monomer used as the second repeating unitrepresented by the above general formula (2) include2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropanerepresented by the following formula (4) and2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]sulfone represented by thefollowing formula (5):

[0205] The above polymer preferably has a polymer molecular weight of10,000 to 1,000,000 at a weight-average molecular weight shown usingpolystyrene conversion. If the above polymer molecular weight is lessthan 10,000, a mechanical strength that is preferable as a polymerelectrolyte membrane might not be obtained. If it exceeds 1,000,000, asdescribed later, when the polymer is dissolved in a solvent to form amembrane, the dissolubility decreases or the viscosity of the solutionincreases, and thereby it becomes difficult to treat the polymer.

[0206] Thereafter, concentrated sulfuric acid is added to the abovepolymer for sulfonation, such that it contains a sulfonic acid groupwithin the range between 0.5 and 3.0 mg equivalent/g. If the obtainedsulfonate contains less than 0.5 mg equivalent/g of sulfonic acid group,it cannot obtain a sufficient ion conductivity. If the content of asulfonic acid group exceeds 3.0 mg equivalent/g, a sufficient toughnesscannot be obtained, and it makes difficult to treat the sulfonate duringthe production of an electrode structure, which will be described later.

[0207] The sulfonate of the above polymer is then dissolved inN-methylpyrrolidone as a solvent to prepare a polymer electrolytesolution. Thereafter, a membrane is formed from the polymer electrolytesolution by the cast method followed by drying in an oven, so as toprepare, for example, the polymer electrolyte membrane 2 having a dryfilm thickness of 50 μm. In the present embodiment, the polymerelectrolyte membrane 2 contains 3 to 15% by weight of the above solventN-methylpyrrolidone, after drying.

[0208] In the present embodiment, the polymer electrolyte membrane 2 issandwiched between the sides of the electrode catalyst layers 1 of theabove electrodes followed by hot pressing, so as to obtain the electrodestructure as shown in FIG. 1. The hot pressing is carried out, forexample, at 150° C. at 2.5 MPa for 1 minute.

[0209] When a separator acting also as a gas passage is furtherlaminated on each of the backing layers 3, 3, the electrode structure inthe present embodiment constitutes a polymer electrolyte fuel cell.

[0210] Next, the present embodiment will be described in the followingexamples and comparative examples.

EXAMPLE 15

[0211] In the present example, first,2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone represented by the aboveformula (3) was polymerized with2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropanerepresented by the above formula (4) at a polymerization ratio of 50:50,so as to obtain a copolymer (n:m=50:50) represented by the followingformula (8):

[0212] Thereafter, concentrated sulfuric acid was added to the abovecopolymer for sulfonation, so as to obtain a sulfonate having an ionexchange capacity of 2.3 meq/g. Thereafter, the sulfonate of the abovecopolymer was dissolved in N-methylpyrrolidone to prepare a polymerelectrolyte solution. A membrane was formed from the polymer electrolytesolution by the cast method followed by drying in an oven, so as toprepare the polymer electrolyte membrane 2 having a dry film thicknessof 50 μm. In the present example, 9 types of the polymer electrolytemembranes 2 were prepared by changing the content of the solvent in theabove membrane after drying within the range of 0 to 30% by weight.

[0213] Subsequently, a platinum particle was supported by carbon black(furnace black) at a certain weight ratio (e.g., carbonblack:platinum=1:1), so as to prepare a catalyst particle. Then, theabove catalyst particles were uniformly dispersed in a perfluoroalkylenesulfonic acid polymer solution (Nafion (trade name) from DuPont) as anion conducting binder solution at a weight ratio of ion conductingbinder:catalyst particle=8:5, so as to prepare a catalyst paste.

[0214] Thereafter, carbon black was mixed with polytetrafluoroethylene(PTFE) particles at a weight ratio of carbon black:PTFE particle=4:6.The obtained mixture was uniformly dispersed in ethylene glycol toobtain a slurry. The obtained slurry was applied on the one side of theabove carbon paper followed by drying it, so as to obtain a substratelayer. Then, two of the backing layers 3 were prepared, each of whichconsisted of the substrate layer and the carbon paper.

[0215] Thereafter, the above catalyst paste was screen printed on eachof the above backing layers 3, so that 0.5 mg/cm² platinum was keptthereon. Then, drying was carried out so as to prepare an electrodecatalyst layer 1. Thus, a pair of electrodes were prepared, each ofwhich consisted of the electrode catalyst layer 1 and the backing layer3. The above drying was carried out by drying at 60° C. for 10 minutesand then vacuum drying at 120° C. for 60 minutes.

[0216] Thereafter, the polymer electrolyte membrane 2 was sandwichedbetween the sides of the electrode catalyst layers 1 of the aboveelectrodes, and hot pressing was then carried out to obtain theelectrode structure as shown in FIG. 1. The hot pressing was carried outby performing the first pressing at 80° C. at 5 MPa for 2 minutes andthen the second pressing at 160° C. at 4 MPa for 1 minute.

[0217] Subsequently, the initial ion conductivity, ion conductivityretention, and toughness of each of the polymer electrolyte membranes 2obtained in the present example were measured.

[0218] The polymer electrolyte membrane 2 was sandwiched between twoplatinum electrodes, and the initial ion conductivity of the membranewas then measured by the alternating two-terminal method (frequency: 10kHz) under the conditions of a temperature of 85° C. and a relativehumidity of 90%. The results are shown in FIG. 7.

[0219] Moreover, the polymer electrolyte membrane 2 was left for 60 daysafter the measurement of the above initial ion conductivity, andthereafter the ion conductivity was measured again by the same method asfor the above initial ion conductivity. The ion conductivity retentionwas calculated as the percentage of the ion conductivity to the aboveinitial ion conductivity. The results are shown in FIG. 8.

[0220] Furthermore, the polymer electrolyte membrane 2 was processed ina dumbbell rated to JIS 7, and the tensile elongation at break wasmeasured under the conditions of a distance between chucks of 20 mm,across head speed of 50 mm/min, a temperature of 25° C. and a relativehumidity of 50%. The obtained tensile elongation at break was defined astoughness. The results are shown in FIG. 9.

[0221]FIGS. 2 and 3 clearly show that if the content of a solvent in thepolymer electrolyte membrane 2 after drying exceeds 15% by weight, theinitial ion conductivity and ion conductivity retention of the membranedrastically decrease, and that if the above content is less than 3% byweight, a good tensile elongation at break cannot be obtained, therebyresulting in low toughness.

[0222] Accordingly, it is clear that when the content of a solvent inthe polymer electrolyte membrane 2 after drying is set within the rangebetween 3 and 15% by weight, the electrode structure of the presentexample comprising the membrane 2 having the above described ionconductivity can have an excellent power generation efficiency.Moreover, it is also clear that when the content of a solvent in thepolymer electrolyte membrane 2 after drying is set within the rangebetween 3 and 15% by weight, the electrode structure of the presentexample comprising the membrane 2 having the above described tensileelongation at break (toughness) can be easily produced.

COMPARATIVE EXAMPLE 10

[0223] In the present comparative example, the sulfonate of the abovecopolymer was dissolved in dimethylacetamide instead ofN-methylpyrrolidone so as to obtain a polymer electrolyte solution, anda membrane was formed from the polymer electrolyte solution by the castmethod. Other than the above exceptions, a polymer electrolyte membranehaving a dry film thickness of 50 μm and a solvent content after dryingof 5% by weight was produced completely in the same manner as in Example15.

[0224] Thereafter, the initial ion conductivity, ion conductivityretention, and toughness of the polymer electrolyte membrane in thepresent comparative example were measured completely in the same manneras in Example 15. The obtained results as well as the results of thepolymer electrolyte membrane 2 in Example 15 whose solvent content afterdrying is 5% by weight are shown in Table 3. TABLE 3 Initial ion IonTensile conductivity conductivity elongation at (S/cm) retention (%)break (%) Example 15 0.19 94.7 21 Comparative 0.20 35.0 19 Example 10

[0225] As is clear from Table 1, the polymer electrolyte membrane formedfrom a polymer electrolyte solution obtained by dissolving the sulfonateof the above copolymer in N-methylpyrrolidone (Example 15) had aninitial ion conductivity and a tensile elongation at break (toughness)substantially equivalent to those of the polymer electrolyte membraneformed from a polymer electrolyte solution obtained by dissolving thesulfonate of the above copolymer in dimethylacetamide (ComparativeExample 10). However, the former was superior to the latter in terms ofion conductivity retention. Accordingly, it is clear that the use of thepolymer electrolyte membrane in Example 15 enables the production of anelectrode structure that has an excellent power generation efficiency.

[0226] Next, the embodiment of the production method of the electrodestructure of the present invention will be explained below.

[0227] As shown in FIG. 1, the electrode structure obtained by theproduction method of the present embodiment comprises a pair ofelectrode catalyst layers 1, 1, a polymer electrolyte membrane 2sandwiched between both the electrode catalyst layers 1, 1, and backinglayers 3, 3 laminated on the electrode catalyst layers 1, 1respectively.

[0228] In the production method in the present embodiment, first thepolymer electrolyte membrane 2 is produced. To produce the membrane 2,for example, 2,5-dichloro-4′-(4-phenoxyphenoxy) benzophenone representedby the following formula (3) is polymerized with2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropanerepresented by the following formula (4) at a polymerization ratio of50:50, so as to obtain a hydrocarbon-based copolymer (n:m=50:50)represented by the following formula (8):

[0229] Then, concentrated sulfuric acid is added to the above copolymerfor sulfonation, such that it contains a sulfonic acid group within therange between 0.5 and 3.0 mg equivalent/g. Thereafter, the sulfonate isdissolved in a solvent such as N-methylpyrrolidone to prepare a polymerelectrolyte solution Thereafter, a membrane is formed from the polymerelectrolyte solution by the cast method followed by drying in an oven,so as to prepare, for example, the polymer electrolyte membrane 2 havinga dry film thickness of 50 μm.

[0230] In the production method in the present embodiment, next,catalyst particles forming the electrode catalyst layer 1 and a catalystpaste comprising the catalyst particles are prepared. The above catalystparticle consists of a platinum particle that is supported by carbonblack (furnace black) at a certain weight ratio (e.g., carbonblack:platinum=1:1). The above catalyst paste is prepared by uniformlymixing the above catalyst particles in an ion conducting polymer bindersolution such as a perfluoroalkylene sulfonic acid polymer (e.g., Nafion(trade name) from DuPont) at a certain weight ratio (e.g., catalystparticle:binder solution=1:1).

[0231] In the production method in the present embodiment, next, thebacking layer 3 is produced. The backing layer 3 consists of a carbonpaper and a substrate layer. The above substrate layer is formed bymixing carbon black and polytetrafluoroethylene (PTFE) particles at acertain weight ratio (e.g., carbon black:PTFE particle=4:6), uniformlydispersing the obtained mixture in a solvent such as ethylene glycol soas to obtain a slurry, and applying the slurry on the one side of theabove carbon paper followed by drying it.

[0232] Thereafter, the above catalyst paste is screen printed on thebacking layer 3, so that a certain amount of catalyst (e.g., 0.5 mg/cm²)is kept thereon, and then drying it, thereby forming the electrodecatalyst layer 1. The catalyst paste screen printed on the backing layer3 is dried, for example, by drying at 60° C. for 10 minutes and thenvacuum drying at 120° C. for 60 minutes.

[0233] Thereafter, the polymer electrolyte membrane 2 is sandwichedbetween a pair of electrode catalyst layers 1, 1 and subjected to hotpressing for integration, so as to obtain the electrode structure asshown in FIG. 1. The hot pressing is carried out, for example, at 150°C. at 2.5 MPa for 1 minute.

[0234] In the production method in the present embodiment, next, anelectric current of 0.1 to 2 A/cm² is applied to the above electrodestructure for 5 hours or more, preferably for 8 hours or more, in ahumidified environment at a relative humidity of 60% or more. As aresult, the electrode catalyst layers 1, 1 penetrate into the polymerelectrolyte membrane 2, so that the length of the interface is extended.Thus, an electrode structure having an excellent adhesion between theelectrode catalyst layers 1, 1 and the polymer electrolyte membrane 2can be obtained.

[0235] As described above, the electrode structure obtained by theproduction method in the present embodiment adopts a structure such thatthe electrode catalyst layers 1, penetrate into the polymer electrolytemembrane 2 thereby extending the length of the interface by the abovedescribed process of supplying an electric current. Thanks to such astructure, in addition to the original function to generate protons andelectrons from reducing gas on the fuel electrode side and to generatewater as a result of the reaction of the above protons with oxidizinggas and electrons on the oxygen electrode side, the electrode catalystlayer 1 has a function to generate water as a result of the reaction ofoxygen gas with hydrogen gas cross leaking out of the polymerelectrolyte membrane 2. As a result, in the electrode structure, bothwater formed in the reaction of the protons with oxidizing gas andelectrons, and water generated due to the above cross leak areefficiently dispersed in the polymer electrolyte membrane 2, therebyproviding the effect of enabling low-humidity operation.

[0236] When a separator acting also as a gas passage is furtherlaminated on each of the backing layers 3, 3, the electrode structureobtained by the production method in the present embodiment constitutesa polymer electrolyte fuel cell.

[0237] Next, the present embodiment will be described in the followingexamples and comparative examples.

EXAMPLE 16

[0238] In the present example, first,2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone represented by the aboveformula (3) was polymerized with2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropanerepresented by the above formula (4) at a molar ratio of 50:50, so as toobtain a copolymer (n:m=50:50) represented by the above formula (8).

[0239] Thereafter, concentrated sulfuric acid was added to the abovecopolymer for sulfonation, so as to obtain a sulfonate having an ionexchange capacity of 2.1 meq/g. Thereafter, the sulfonate of the abovecopolymer was dissolved in N-methylpyrrolidone to prepare a polymerelectrolyte solution. A membrane was formed from the polymer electrolytesolution by the cast method followed by drying in an oven, so as toprepare the polymer electrolyte membrane 2 having a dry film thicknessof 50 μm.

[0240] Subsequently, a platinum particle was supported by carbon black(furnace black) at a weight ratio of carbon black:platinum=1:1, so as toprepare a catalyst particle. Then, the above catalyst particles wereuniformly dispersed in a solution containing a perfluoroalkylenesulfonic acid polymer (Nafion (trade name) from DuPont) as an ionconducting polymer binder at a weight ratio of binder:carbon black=1:1,so as to prepare a catalyst paste.

[0241] Thereafter, carbon black was mixed with polytetrafluoroethylene(PTFE) particles at a weight ratio of carbon black:PTFE particle=4:6.The obtained mixture was uniformly dispersed in a solvent such asethylene glycol to obtain a slurry. The obtained slurry was applied onthe one side of the above carbon paper followed by drying it, so as toobtain a substrate layer. Then, two of the backing layers 3 wereprepared, each of which consisted of the substrate layer and the carbonpaper.

[0242] Thereafter, the above catalyst paste was screen printed on eachof the above backing layers 3, so that 0.5 mg/cm² platinum was keptthereon. Then, drying was carried out so as to prepare an electrodecatalyst layer 1. Thus, a pair of electrodes were prepared, each ofwhich consisted of the electrode catalyst layer 1 and the backing layer3. Thereafter, the polymer electrolyte membrane 2 was sandwiched betweenthe sides of the electrode catalyst layers 1 of the above electrodes,and hot pressing was then carried out for integration, so as to obtainthe electrode structure as shown in FIG. 1.

[0243] Thereafter, an electric current of 1 A/cm² was applied to theabove electrode structure for 18 hours in a humidified environment at arelative humidity of 100%, so as to complete the electrode structure.

[0244] Thereafter, the electrode structure obtained in the presentexample was used for a single cell. Air was supplied to the oxygenelectrode side, whereas pure hydrogen was supplied to the fuel electrodeside, so as to generate electric power. A cell potential at a currentdensity of 1 A/cm² was measured as a generated electric potential. Powergeneration conditions were a temperature of 80° C., a relative humidityof 80% on the oxygen electrode side, and a relative humidity of 50% onthe fuel electrode side. The results are shown in Table 4.

EXAMPLE 17

[0245] In the present example, the polymer electrolyte membrane 2 wassandwiched between the sides of the electrode catalyst layers 1 of theelectrodes, and hot pressing was then carried out to obtain a integratedelectrode structure. Then, an electric current of 0.8 A/cm² was appliedto the thus obtained electrode structure for 24 hours in a humidifiedenvironment at a relative humidity of 80%. Other than the aboveexception, an electrode structure was completed completely in the samemanner as in Example 16.

[0246] Thereafter, the electrode structure obtained in the presentexample was used for a single cell, and the generated electric potentialwas measured completely in the same manner as in Example 16. The resultsare shown in Table 4.

EXAMPLE 18

[0247] In the present example, the polymer electrolyte membrane 2 wassandwiched between the sides of the electrode catalyst layers 1 of theelectrodes, and hot pressing was then carried out to obtain a integratedelectrode structure. Then, an electric current of 0.3 A/cm² was appliedto the thus obtained electrode structure for 16 hours in a humidifiedenvironment at a relative humidity of 85%. Other than the aboveexception, an electrode structure was completed completely in the samemanner as in Example 16.

[0248] Thereafter, the electrode structure obtained in the presentexample was used for a single cell, and the generated electric potentialwas measured completely in the same manner as in Example 16. The resultsare shown in Table 4.

EXAMPLE 19

[0249] In the present example, the polymer electrolyte membrane 2 wassandwiched between the sides of the electrode catalyst layers 1 of theelectrodes, and hot pressing was then carried out to obtain a integratedelectrode structure. Then, an electric current of 0.9 A/cm² was appliedto the thus obtained electrode structure for 24 hours in a humidifiedenvironment at a relative humidity of 60%. Other than the aboveexception, an electrode structure was completed completely in the samemanner as in Example 16.

[0250] Thereafter, the electrode structure obtained in the presentexample was used for a single cell, and the generated electric potentialwas measured completely in the same manner as in Example 16. The resultsare shown in Table 4.

EXAMPLE 20

[0251] In the present example, the polymer electrolyte membrane 2 wassandwiched between the sides of the electrode catalyst layers 1 of theelectrodes, and hot pressing was then carried out to obtain a integratedelectrode structure. Then, an electric current of 0.15 A/cm² was appliedto the thus obtained electrode structure for 24 hours in a humidifiedenvironment at a relative humidity of 100%. Other than the aboveexception, an electrode structure was completed completely in the samemanner as in Example 16.

[0252] Thereafter, the electrode structure obtained in the presentexample was used for a single cell, and the generated electric potentialwas measured completely in the same manner as in Example 16. The resultsare shown in Table 4.

COMPARATIVE EXAMPLE 11

[0253] In the present comparative example, an electrode structure wascompleted completely in the same manner as in Example 16 with theexception that the process of applying an electric current in ahumidified environment was not carried out at all on the electrodestructure integrated by sandwiching the polymer electrolyte membrane 2between the sides of the electrode catalyst layers 1 of the electrodesand performing hot pressing to it.

[0254] Thereafter, the electrode structure obtained in the presentcomparative example was used for a single cell, and its generatedelectric potential was measured completely in the same manner as inExample 16. The results are shown in Table 4.

COMPARATIVE EXAMPLE 12

[0255] In the present comparative example, an electrode structure wascompleted completely in the same manner as in Example 16 with theexception that an electric current of 0.5 A/cm² was applied to theelectrode structure, which was integrated by sandwiching the polymerelectrolyte membrane 2 between the sides of the electrode catalystlayers 1 of the electrodes and performing hot pressing to it, for 12hours in a humidified environment at a relative humidity of 50%.

[0256] Thereafter, the electrode structure obtained in the presentcomparative example was used for a single cell, and the generatedelectric potential was measured completely in the same manner as inExample 16. The results are shown in Table 4.

COMPARATIVE EXAMPLE 13

[0257] In the present comparative example, an electrode structure wascompleted completely in the same manner as in Example 16 with theexception that no electric current was applied to the electrodestructure that was integrated by sandwiching the polymer electrolytemembrane 2 between the sides of the electrode catalyst layers 1 of theelectrodes and performing hot pressing to it, but the electrodestructure was retained for 12 hours in a humidified environment at arelative humidity of 90%.

[0258] Thereafter, the electrode structure obtained in the presentcomparative example was used for a single cell, and the generatedelectric potential was measured completely in the same manner as inExample 16. The results are shown in Table 4. TABLE 4 Generated RelativeElectric electric humidity current potential (%) (A/cm²) (V) EvaluationExample 16 100 1.00 0.62 G Example 17 80 0.80 0.58 G Example 18 85 0.300.56 G Example 19 60 0.90 0.61 G Example 20 100 0.15 0.56 G ComparativeNot Not 0.47 P Example 11 humidified applied Comparative 50 0.50 0.50 IExample 12 Comparative 90 Not 0.48 p Example 13 applied

[0259] Table 4 clearly shows that the electrode structures of Examples16 to 20 in which an electric current of 0.15 to 1 A/cm² was applied tothe electrode structure for 16 to 24 hours in a humidified environmentat a relative humidity of 60% or higher after the electrode catalystlayers 1, 1 were integrated with the polymer electrolyte membrane 2 issuperior in a power generation efficiency than the electrode structureof Comparative Example 11 in which the above process was not carried outat all, and that they are also superior in adhesion between theelectrode catalyst layers 1, 1 and the polymer electrolyte membrane 2.

[0260] Moreover, even when compared with the electrode structure ofComparative Example 12 in which an electric current of 0.5 A/cm² wasapplied thereto for 12 hours but it was carried out under a humidifiedcondition of a relative humidity of less than 60%, or the electrodestructure of Comparative Example 13 in which it was retained for 12hours in a humidified environment at a relative humidity of 90% but noelectric current was applied thereto, the electrode structures ofExamples 16 to 20 have an excellent power generation efficiency, andalso have an excellent adhesion between the electrode catalyst layers 1,1 and the polymer electrolyte membrane 2.

Industrial Applicability

[0261] The present invention can be used for a polymer electrolyte fuelcell, which is used in vehicles and the like.

1. An electrode structure for a polymer electrolyte fuel cell comprisinga pair of electrode catalyst layers and a polymer electrolyte membranesandwiched between the electrode catalyst layers, wherein said polymerelectrolyte membrane is a sulfonate of a hydrocarbon-based polymercomprising a main chain, in which a plurality of benzene rings are boundto one another, directly or through a divalent organic group.
 2. Theelectrode structure for a polymer electrolyte fuel cell according toclaim 1, wherein said polymer electrolyte membrane contains 5% or moreby weight of the water coordinated to protons of sulfonic acid groupsbased on the total weight of the polymer electrolyte membrane.
 3. Theelectrode structure for a polymer electrolyte fuel cell according toclaim 1, wherein said polymer electrolyte membrane comprises an ionconducting polymer containing fluorine in a molecular structure thereof,and the ratio (Y/X) of the fluorine content in said polymer electrolytemembrane (Y) to the fluorine content in said electrode catalyst layer(X) is within the range of 0.2 to 2.0.
 4. The electrode structure for apolymer electrolyte fuel cell according to claim 3, wherein that saidpolymer electrolyte membrane consists of a sulfonate of a copolymer of afirst repeating unit represented by the following general formula (1)and a second repeating unit represented by the following general formula(2), and in that the first repeating unit or the second repeating unitcontains fluorine:

wherein A represents an electron attracting group, B represents anelectron releasing group group, n is an integer of 0 or 1, and a benzenering includes a derivative thereof, and

wherein A represents an electron attracting group, B represents anelectron releasing group group, Y represents —C(CF₃)₂— or —SO₂—, and abenzene ring includes a derivative thereof.
 5. The electrode structurefor a polymer electrolyte fuel cell according to claim 1, wherein thatsaid polymer electrolyte membrane consists of a sulfonate of a copolymerof a first repeating unit represented by the following general formula(1) and a second repeating unit represented by the following generalformula (2), and in that said electrode catalyst layer contains, as acatalyst, platinum within the range of from 0.01 to 0.8 mg/cm², and theaverage diameter of a carbon particle as a carrier supporting theplatinum is within the range of from 10 to 100 nm:

 wherein A represents an electron attracting group, B represents anelectron releasing group group, n is an integer of 0 or 1, and a benzenering includes a derivative thereof, and

 wherein A represents an electron attracting group, B represents anelectron releasing group group, Y represents —C(CF₃)₂— or —SO₂—, and abenzene ring includes a derivative thereof.
 6. The electrode structurefor a polymer electrolyte fuel cell according to claim 5, wherein saidcopolymer comprises from 10 to 80 mol % of said first repeating unit andfrom 90 to 20 mol % of said second repeating unit.
 7. The electrodestructure for a polymer electrolyte fuel cell according to claim 5 or 6,wherein the sulfonate of said copolymer contains a sulfonic acid groupwithin the range of from 0.5 to 3.0 mg equivalent/g.
 8. The electrodestructure for a polymer electrolyte fuel cell according to claim 1,wherein said polymer electrolyte membrane is produced by forming amembrane from a solution obtained by dissolving into a solvent asulfonate of a copolymer of a first repeating unit represented by thefollowing general formula (1) and a second repeating unit represented bythe following general formula (2) and drying the obtained membrane, andthe polymer electrolyte membrane contains from 3 to 15% by weight ofsaid solvent after drying:

wherein A represents an electron attracting group, B represents anelectron releasing group group, n is an integer of 0 or 1, and a benzenering includes a derivative thereof, and

 wherein A represents an electron attracting group, B represents anelectron releasing group group, Y represents —C(CF₃)₂— or —SO₂—, and abenzene ring includes a derivative thereof.
 9. The electrode structurefor a polymer electrolyte fuel cell according to claim 8, wherein saidsolvent is N-methylpyrrolidone.
 10. The electrode structure for apolymer electrolyte fuel cell according to claim 8 or 9, characterizedin that said copolymer comprises from 10 to 80 mol % of said firstrepeating unit and from 90 to 20 mol % of said second repeating unit.11. The electrode structure for a polymer electrolyte fuel cellaccording to any one of claims 8 to 10, wherein said copolymer containssulfonic acid groups within the range from 0.5 to 3.0 mg equivalent/g.12. A method of producing an electrode structure for a polymerelectrolyte fuel cell, comprising the steps of: holding a polymerelectrolyte membrane between a pair of electrode catalyst layers tointegrate both the electrode catalyst layers and the polymer electrolytemembrane, so as to form an electrode structure; and applying an electriccurrent of 0.1 A/cm² or higher to the electrode structure for 5 hours ormore in a humidified environment at a relative humidity of 60% or more.13. The method of producing an electrode structure for a polymerelectrolyte fuel cell according to claim 12, characterized in that saidpolymer electrolyte membrane is a sulfonate of a hydrocarbon-basedpolymer comprising a main chain, in which two or more benzene rings arebound to one another, directly or through a divalent organic group. 14.A polymer electrolyte fuel cell, characterized in that it comprises anelectrode structure comprising a pair of electrode catalyst layers and apolymer electrolyte membrane held between the electrode catalyst layers,wherein said polymer electrolyte membrane is a sulfonate of ahydrocarbon-based polymer comprising a main chain, in which two or morebenzene rings are bound to one another, directly or through a divalentorganic group, and in that said polymer electrode fuel cell generatespower, when oxidizing gas is supplied to the one side of said electrodestructure and reducing gas to the other side.
 15. The polymer electrodefuel cell according to claim 14, characterized in that said electrodecatalyst layer comprises a carbon particle supporting a catalystparticle that is integrated by an ion conducting polymer bindercontaining fluorine in a molecular structure thereof, in that saidpolymer electrolyte membrane comprises an ion conducting polymercontaining fluorine in a molecular structure thereof, and the ratio(Y/X) of the fluorine content in said polymer electrolyte membrane (Y)to the fluorine content in said electrode catalyst layer (X) is withinthe range of from 0.2 to 2.0.
 16. The polymer electrode fuel cellaccording to claim 14, wherein said polymer electrolyte membrane is asulfonate of a copolymer of a first repeating unit represented by thefollowing general formula (1) and a second repeating unit represented bythe following general formula (2), said electrode catalyst layercontains, as a catalyst, platinum within the range or from 0.01 to 0.8mg/cm², and the average diameter of a carbon particle as a carriersupporting the platinum is within the range or from 10 to 100 nm:

wherein A represents an electron attracting group, B represents anelectron releasing group group, n is an integer of 0 or 1, and a benzenering includes a derivative thereof, and

wherein A represents an electron attracting group, B represents anelectron releasing group group, Y represents —C(CF₃)₂— or —SO₂—, and abenzene ring includes a derivative thereof.
 17. The polymer electrodefuel cell according to claim 14, wherein said polymer electrolytemembrane is produced by forming a membrane from a solution obtained bydissolving into a solvent a sulfonate of a copolymer of a firstrepeating unit represented by the following general formula (1) and asecond repeating unit represented by the following general formula (2)and drying the obtained membrane, and in that the membrane contains from3 to 15% by weight of said solvent after drying:

wherein A represents an electron attracting group, B represents anelectron releasing group group, n is an integer of 0 or 1, and a benzenering includes a derivative thereof, and

wherein A represents an electron attracting group, B represents anelectron releasing group group, Y represents —C(CF₃)₂— or —SO₂—, and abenzene ring includes a derivative thereof.
 18. The polymer electrodefuel cell according to claim 14, wherein said electrode structure isformed by holding a polymer electrolyte membrane between a pair ofelectrode catalyst layers to integrate both the electrode catalystlayers and the polymer electrolyte membrane, and applying an electriccurrent of 0.1 A/cm² or higher to integrated body of the electrodecatalyst layers and the polymer electrolyte membrane for 5 hours or morein a humidified environment at a relative humidity of 60% or more.